Rail stack array of charge storage devices and method of making same

ABSTRACT

There is provided a monolithic three dimensional array of charge storage devices which includes a plurality of device levels, wherein at least one surface between two successive device levels is planarized by chemical mechanical polishing.

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/801,233, filed on Mar. 6, 2001, which is acontinuation-in-part of U.S. application Ser. No. 09/745,125, filed onDec. 22, 2000, both of which are incorporated by reference in theirentirety. This application is also a continuation-in-part of U.S.application Ser. No. 09/639,579 filed on Aug. 14, 2000, which isincorporated by reference in its entirety. This application is also acontinuation-in-part of U.S. application Ser. No. 09/639,702 filed onAug. 14, 2000, which is incorporated by reference in its entirety. Thisapplication is also a continuation-in-part of U.S. application Ser. No.09/639,749 filed on Aug. 14, 2000, which is incorporated by reference inits entirety. This application also claims benefit of priority ofprovisional application 60/279,855 filed on Mar. 28, 2001, which isincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to semiconductor devices in generaland to a three dimensional TFT array in particular.

[0004] 2. Discussion of Related Art

[0005] As integrated circuits and computers have become powerful, newapplications have arisen that require the ability to store large amountsof data. Certain applications require a memory with the ability to writeand erase data and the ability to store data in a nonvolatile manner.There are many applications which can be enabled by bringing the priceper megabyte of semiconductor memory down well below a dollar (US) permegabyte so that it becomes price competitive with, for example: (1)chemical film for the storage of photographic images; (2) Compact Disks(CDs) for the storage of music and textual data for distribution; (3)Digital Versatile Disks (DVDs) for the storage of video and multi-mediamaterials for distribution; and (4) Video Tape and Digital Audio andVideo Tape for the storage of consumer audio and video recordings. Suchmemories should be archival and non-volatile in that they should be ableto withstand being removed from equipment and all sources of power for aperiod of up to about 10 years with no significant degradation of theinformation stored in them. Such a requirement approximates the typicallongevity for CDs, DVDs, magnetic tape and most forms of photographicfilm.

[0006] Presently, such memories are formed with electrically erasablenonvolatile memories such as flash memories and EEPROMs. Unfortunately,these devices are typically fabricated in a single crystalline siliconsubstrate and therefore are limited to two-dimensional arrays of storagedevices, thereby limiting the amount of data that can be stored to thenumber of devices that can be fabricated in a single plane of silicon.

[0007] It has also been known to fabricate nonvolatile memories thatemployed trapped charge in a dielectric layer. Typically, electrons aretrapped in a layer of silicon nitride by, for instance, tunneling acurrent through the nitride layer. The silicon nitride is formed betweena gate insulated from the channel of a field-effect transistor. Thetrapped charge shifts the threshold voltage of the transistor and thus,the threshold voltage is sensed to determine whether or not charge istrapped in the nitride layer. See U.S. Pat. No. 5,768,192 for an exampleof such memories.

[0008] U.S. Pat. No. 5,768,192, issued to B. Eitan, and the technicalarticle entitled “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell” by B. Eitan et al. in IEEE Electron Device Letters, vol.21, No. 11, November 2000, pp. 543-545 teach a nonvolatile semiconductormemory cell which uses asymmetrical charge trapping in the nitridecharge storage layer of the Oxide-Nitride-Oxide (ONO) stack to store twobits in one cell. The cell is written by hot electron injection into thecharge storage layer above the drain junction. The cell is read in theopposite direction to which it was written, i.e., voltages are appliedto the source and gate, with the drain grounded. The memory cell isconstructed in a p− type silicon substrate. However, thissilicon-oxide-nitride-oxide-silicon (SONOS) 1 TC memory is arranged inan NOR Virtual Ground Array with a cell area of 2.5F² per bit, where Fis the minimum feature size. This cell area is larger than desirable,and leads to a less than optimum cell density.

[0009] Prior art negative-resistance devices are also known. Thesedevices were discovered around 1972 and are described inThin-MIS-Structure Si Negative-Resistance Diode, Applied PhysicsLetters, Volume 20, No. 8, beginning on page 269, 15 Apr. 1972. Thedevice described in the article is a junction diode, such as diode 5510of FIG. 96 and a thin oxide region disposed on the n− type region of thediode, such as the oxide region 5511 of FIG. 96. The device provides aswitching phenomenon exhibiting a negative-resistance region as shown inFIG. 97. Note as the potential on the diode is increased in the diode'sforward direction, little conduction occurs until the voltage firstreaches the voltage shown as point 5512 at which point the deviceexhibits a negative-resistance. From there the device exhibits asomewhat diode-like characteristic as shown by the segment 5513 in FIG.97. This switching characteristic is used to fabricate static memorycells (flip-flops) such as shown in U.S. Pat. Nos. 5,535,156 and6,015,738. Additionally, the basic operation of this device is describedin Sze's, The Physics of Semiconductor Devices, (2^(nd) edition, Chapter9.5, pp. 549-553), although this explanation may contain an error in itsdiscussion in polarity.

[0010] The device of FIG. 96 comprises a PN junction diode and a thinoxide region. When the diode is forward biased, initially very littlecurrent flows because the diode junction voltage is a fraction of theapplied voltage, with the balance of the voltage drop across the n−region and oxide region. Holes injected into the n− region from the pregion are sufficiently low in number that the tunneling current throughthe oxide (despite the unfavorable barrier to the hole flow) allows then− region to remain an n− type region. Similarly, any holes generatedwithin the depletion region are able to pass through the thin oxidewhile any generated electrons are swept across to the p region and outof the anode contact.

[0011] As the applied forward voltage increases, the n− region begins todeplete at the interface with the oxide just as in a normal MOSFET asthe threshold voltage is approached. At a high enough voltage, thisdepletion region extends all the way to the junction to producepunch-through, resulting in a significant injection of holes from the pregion into the n− layer. The holes cannot flow well through the oxideand consequently build up near the surface. This causes the n− region toinvert more strongly near the oxide interface, and increasing thevoltage drop across the oxide, recalling that V=Q/C. The electrontunneling current through the oxide rises by a super-exponential factor,increasing the forward bias across the diode and the current. At thesame time holes flood the n− region, raising its conductivity andreducing its voltage drop. Since the voltage across the diode isrelatively small (and changes little, even for large changes in current)a large reduction in the n− voltage drop reduces the voltage across theentire structure dramatically (assuming a suitable series resistance inthe circuit to avoid device rupture). Thus, the regenerative action ofthe foregoing description causes a rapid increase in current,accompanied by a rapid decrease in voltage. It is thisnegative-resistance region that has been exploited to make the SRAMcells described in the above referenced patents.

[0012] At higher current levels, the device behaves essentially as anordinary forward biased diode as most of the voltage is ultimatelydropped across the PN junction. Overall, the V-I characteristics of thestructure are shown in FIG. 97 with the slope of the segment 5513 beingdetermined in large part by the series resistance coupled to thestructure of FIG. 96.

[0013] When reverse biased, the diode is in its blocking state and theonly current that flows through the oxide is electron leakage current.The reverse junction voltage is a fraction of the applied voltagebecause some is dropped across the oxide region. It should be noted thatelectrons carry current through the oxide region in both reverse biasand in a strong forward bias.

[0014] Another type of prior art memory device is disclosed in thetechnical article entitled “A Novel Cell Structure for Giga-bit EPROMsand Flash Memories Using Polysilicon Thin Film Transistors” by S. Koyamain 1992 Symposium on VLSI Technology Digest of Technical Papers, pp.44-45. As shown in FIG. 98, each memory cell is a “self-aligned”floating gate cell and contains a polycrystalline silicon thin filmtransistor electrically erasable programmable read only memory (TFTEEPROM) over an insulating layer. In this device, the bit lines extendin the direction parallel to the source-channel-drain direction (i.e.,the bit lines extend parallel to the charge carrier flow direction). Theword lines extend in the direction perpendicular to thesource-channel-drain direction (i.e., the word lines extendperpendicular to the charge carrier flow direction). The TFT EEPROMs donot contain a separate control gate. Instead, the word line acts as acontrol gate in regions where it overlies the floating gates.

[0015] The layout of Koyama requires two polycide contact pads to beformed to contact the source and drain regions of each TFT. The bitlines are formed above the word lines and contact the contact padsthrough contact vias in an interlayer insulating layer which separatesthe bits lines from the word lines. Therefore, each cell in this layoutis not fully self-aligned, because the contact pads and the contact viasare each patterned using a non-self aligned photolithography step.Therefore, each memory cell has an area that is larger than desirable,and leads to a less than optimum cell density. The memory cell of Koyamais also complex to fabricate because it requires the formation ofcontact pads and bit line contact vias. Furthermore, themanufacturability of the device of Koyama is less than optimum becauseboth bit lines and word lines have a non-planar top surface due to thenon-planar underlying topography. This may lead to open circuits in thebit and word lines.

[0016] The Virtual Ground Array approach to crystalline siliconnon-volatile memories has also been known for some time and is anelegant way of aggressively reducing memory cell size. Turning now toFIG. 99, the basic approach utilizes a cross point array 5610 ofbitlines in buried n+ diffusion 5612 within a single crystalline siliconp− type substrate 5614 and wordlines formed of polysilicon rails 5616disposed over the substrate 5614. A transistor is formed from adjacentbitlines 5612 and a p− type channel region 5618 disposed between theadjacent bitlines 5612. A layer of gate oxide 5620 insulates thefloating gates 5622, which lie above the channels 5618 and are formedof, for example, polysilicon. An upper dielectric layer 5624 insulatesthe floating gates 5622 from polysilicon wordlines (WLs) 5616.

[0017] “Virtual Ground” refers to the fact that there is no dedicatedground line in the array. Whenever a cell is chosen for read or program,a pair of buried n+ bitlines (BLs) is the source and drain with thesource grounded. For example, to select the cell 5624 outlined in FIG.100, BL(k) and BL(k+1) would be selected as the source and drain (orvice versa) and WL(j) would be selected as the control gate of thedevice. In one approach, all of the bit lines to the left of BL(k) asshown in FIG. 100 would be held at the same potential as BL(k) and allof the bit lines to the right of BL(k+1) would be held at the samepotential as BL(k+1) so that source-drain current would only flow (forread and programming) in the selected cell (all other WLs beinggrounded).

[0018] In all of these approaches, the charge storage medium is aconducting floating gate made of doped polysilicon. By hot electroninjection programming (the method of choice in all classic EPROM(erasable programmable read only memory) and single transistor Flashmemory cells), electrons are injected onto the floating gate thuschanging the threshold voltage of the inherent MOS transistor.

[0019] The above discussed SONOS (polysilicon-blockingoxide-nitride-tunnel oxide-silicon) charge trapping approach hasreemerged as a viable candidate for non-volatile MTP memories arrangedin a virtual ground array structure 5626, as shown in FIG. 101. Thearray includes n+ buried bitlines 5612 disposed in a single crystallinesilicon substrate 5614. An ONO (oxide-nitride-oxide) dielectric stack5628 insulates bitlines 5612 from polysilicon wordline 5630. The hotelectrons are injected into the ONO dielectric stack 5628 near the drainedge during programming where charge is trapped in the nitride layer.Two bits can be stored per memory cell utilizing this approach becausehot electrons are injected into the ONO dielectric stack at theprogramming drain edge. Since the nitride charge storage medium does notlaterally conduct, the charge stays where it was injected. Trappedcharge near the source of a transistor has a large effect on thetransistor's threshold voltage while trapped charge near the drain haslittle effect on threshold voltage. Accordingly, individual charge zoneson either side of the ONO layer may be written and read by simplyreversing the drain and source connections for the cell. When the cellis programmed, charge is injected at the zone closest to the drain. Ifsource and drain are reversed for the same cell, another charge may beinjected into the same cell but at the “other” drain. Both sides canalso be read, thus two bits per cell may be stored and retrieved.

[0020] The above described prior art devices are relatively expensivebecause their density is not optimized.

SUMMARY OF THE INVENTION

[0021] According to one preferred embodiment of the present invention, asemiconductor device comprises a monolithic three dimensional array ofcharge storage devices comprising a plurality of device levels, whereinat least one surface between two successive device levels is planarizedby chemical mechanical polishing.

[0022] In another preferred embodiment of the present invention, amonolithic three dimensional array of charge storage devices is formedin an amorphous or polycrystalline semiconductor layer over amonocrystalline semiconductor substrate, and driver circuitry is formedin the substrate at least in part under the array, within the array orabove the array.

[0023] Another preferred embodiment of the present invention provides amemory device comprising a first input/output conductor formed above oron a first plane of a substrate. The memory device also includes asecond input/output conductor. A semiconductor region is located betweenthe first input/output conductor and the second input/output conductorat an intersection of their projections. The memory device includes acharge storage medium wherein charge stored in the charge storage mediumaffects the amount of current that flows between the first input/outputconductor and the second input/output conductor.

[0024] Another preferred embodiment of the present invention provides anonvolatile read-write memory cell having an N doped region, a P dopedregion, and a storage element disposed between the two.

[0025] Another preferred embodiment of the present invention provides amethod for operating a memory cell. The method comprises the steps oftrapping charge in a region to program the cell, and passing currentthrough the region when reading data from the cell.

[0026] Another preferred embodiment of the present invention provides anarray of memory cells, said array having a plurality of memory cellseach comprising at least one semiconductor region and a storage meansfor trapping charge. The array also has control means for controllingthe flow of current through the semiconductor region and the storagemeans of the cells.

[0027] Another preferred embodiment of the present invention provides anonvolatile stackable pillar memory device and its method offabrication. The memory device includes a substrate having a firstplane. A first contact is formed on or above the plane of a substrate. Abody is formed on the first contact. A second contact is formed on thebody wherein the second contact is at least partially aligned over thefirst contact. A control gate is formed adjacent to the charge storagemedium. A read current flows between the first contact and the secondcontact in a direction perpendicular to the plane of the substrate.

[0028] Another preferred embodiment of the present invention provides afield effect transistor, comprising a source, a drain, a channel, agate, at least one insulating layer between the gate and the channel,and a gate line which extends substantially parallel to asource-channel-drain direction and which contacts the gate and is selfaligned to the gate.

[0029] Another preferred embodiment of the present invention provides athree dimensional nonvolatile memory array, comprising a plurality ofvertically separated device levels, each level comprising an array ofTFT EEPROMs, each TFT EEPROM comprising a channel, source and drainregions, and a charge storage region adjacent to the channel, aplurality of bit line columns in each device level, each bit linecontacting the source or the drain regions of the TFT EEPROMs, aplurality of word line rows in each device level, and at least oneinterlayer insulating layer located between the device levels.

[0030] Another preferred embodiment of the present invention provides anEEPROM comprising a channel, a source, a drain, a tunneling dielectriclocated above the channel, a floating gate located above the tunnelingdielectric, sidewall spacers located adjacent to the floating gatesidewalls, a word line located above the floating gate, and a controlgate dielectric located between the control gate and the floating gate.The control gate dielectric is located above the sidewall spacers.

[0031] Another preferred embodiment of the present invention provides anarray of nonvolatile memory cells, wherein each memory cell comprises asemiconductor device and each memory cell size per bit is about (2F²)/N,where F is a minimum feature size and N is a number of device layers inthe third dimension, and where N>1 Another preferred embodiment of thepresent invention provides a method of making an EEPROM, comprisingproviding a semiconductor active area, forming a charge storage regionover the active area, forming a conductive gate layer over the chargestorage region and patterning the gate layer to form a control gateoverlying the charge storage region. The method also comprises dopingthe active area using the control gate as a mask to form source anddrain regions in the active area, forming a first insulating layer aboveand adjacent to the control gate, exposing a top portion of the controlgate without photolithographic masking, and forming a word linecontacting the exposed top portion of the control gate, such that theword line is self aligned to the control gate.

[0032] Another preferred embodiment of the present invention provides amethod of making an EEPROM, comprising providing a semiconductor activearea, forming a tunnel dielectric layer over the active area, forming aconductive gate layer over the tunnel dielectric layer, patterning thegate layer to form a floating gate overlying the tunnel dielectric layerand doping the active area using the floating gate as a mask to formsource and drain regions in the active area. The method also comprisesforming sidewall spacers adjacent to the floating gate sidewalls,forming a first insulating layer above and adjacent to the sidewallspacers and above the source and drain regions, forming a control gatedielectric layer over the floating gate, and forming a word line overthe control gate dielectric and over the first insulating layer.

[0033] Another preferred embodiment of the present invention provides amethod of forming a nonvolatile memory array, comprising forming asemiconductor active layer, forming a first insulating layer over theactive layer, forming a plurality of gate electrodes over the firstinsulating layer and doping the active layer using the gate electrodesas a mask to form a plurality of source and drain regions in the activelayer, and a plurality of bit lines extending substantiallyperpendicular to a source-drain direction. The method also comprisesforming a second insulating layer above and adjacent to the gateelectrodes and above the source regions, drain regions and the bitlines, planarizing the second insulating layer, and forming a pluralityof word lines over the second insulating layer extending substantiallyparallel to the source-drain direction.

[0034] Another preferred embodiment of the present invention provides amethod of making an EEPROM array, comprising providing a semiconductoractive area, forming a plurality of dummy blocks above the active area,doping the active area using the dummy blocks as a mask to form sourceand drain regions in the active area, forming an intergate insulatinglayer above and between the dummy blocks, planarizing the intergateinsulating layer to expose top portions of the dummy blocks, selectivelyremoving the dummy blocks from between portions of the planarizedintergate insulating layer to form a plurality of vias between theportions of the intergate insulating layer, forming charge storageregions over the active area in the plurality of vias, forming aconductive gate layer over the charge storage regions, and patterningthe conductive gate layer to form a control gate overlying the chargestorage region.

[0035] Another preferred embodiment of the present invention provides amethod of forming a TFT EEPROM, comprising forming a TFT EEPROMcomprising an amorphous silicon or a polysilicon active layer, a chargestorage region and a control gate, providing a crystallization catalystin contact with the active layer, and heating the active layer after thestep of providing the catalyst to recrystallize the active layer usingthe catalyst.

[0036] Another preferred embodiment of the present invention provides atwo- or three-dimensional memory array constructed of thin filmtransistors disposed above the substrate. Spaced-apart conductorsdisposed in a first direction form contacts with memory cells formed inrail stacks disposed in a second direction different from the firstdirection. A local charge trapping medium receives and stores hotelectrons injected by thin film transistors formed at the intersectionsof the spaced-apart conductors and the rail stacks. The local chargetrapping medium may be used to store charge adjacent to a transistordrain and by reversing the drain and source lines, two bits per memorycell may be stored, if desired. A programming method insures that storedmemory will not be inadvertently disturbed.

[0037] Another preferred embodiment of the present invention provides anon-volatile thin film transistor (TFT) memory device that isconstructed above a substrate. It employs a source, drain and channelformed of transition metal crystallized silicon. A local charge storagefilm is disposed vertically adjacent to the channel and stores injectedcharge. A two- or three-dimensional array of such devices may beconstructed above the substrate. Spaced-apart conductors disposed in afirst direction form contacts with memory cells formed in rail stacksdisposed in a second direction different from the first direction. Thelocal charge storage film receives and stores charge injected by TFTsformed at the intersections of the spaced-apart conductors and the railstacks. The local charge storage film may be used to store chargeadjacent to a transistor drain and by reversing the drain and sourcelines, two bits per memory cell may be stored, if desired. A programmingmethod insures that stored memory will not be inadvertently disturbed.

[0038] Another preferred embodiment of the present invention provides aflash memory array disposed above a substrate, the array comprising afirst plurality of spaced-apart conductive bit lines disposed at a firstheight above the substrate in a first direction, and a second pluralityof spaced-apart rail-stacks disposed at a second height in a seconddirection different from the first direction, each rail-stack includinga plurality of semiconductor islands whose first surface is in contactwith said first plurality of spaced-apart conductive bit lines, aconductive word line, and charge storage regions disposed between asecond surface of the semiconductor islands and the word line.

[0039] Another preferred embodiment of the present invention provides aTFT CMOS device, comprising a gate electrode, a first insulating layeradjacent to a first side of the gate electrode, a first semiconductorlayer having a first conductivity type disposed on a side of the firstinsulating layer opposite to the gate electrode, a first source anddrain regions of a second conductivity type disposed in the firstsemiconductor layer, first source and drain electrodes in contact withthe first source and drain regions and disposed on a side of the firstsemiconductor layer opposite to the first insulating layer. The TFT CMOSdevice further comprises a second insulating layer adjacent to a secondside of the gate electrode, a second semiconductor layer having a secondconductivity type disposed on a side of the second insulating layeropposite to the gate electrode, second source and drain regions of afirst conductivity type disposed in the second semiconductor layer, andsecond source and drain electrodes in contact with the second source anddrain regions and disposed on a side of the second semiconductor layeropposite to the second insulating layer.

[0040] Another preferred embodiment of the present invention provides acircuit comprising a plurality of charge storage devices and a pluralityof antifuse devices.

[0041] Another preferred embodiment of the present invention provides asemiconductor device comprising a semiconductor active region, a chargestorage region adjacent to the semiconductor active region, a firstelectrode, and a second electrode. Charge is stored in the chargestorage region when a first programming voltage is applied between thefirst and the second electrodes, and a conductive link is formed throughthe charge storage region to form a conductive path between the firstand the second electrodes when a second programming voltage higher thanthe first voltage is applied between the first and the secondelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1A is an illustration of a pillar memory in accordance withan embodiment of the present invention.

[0043]FIG. 1B is an illustration of an overhead view of a pillar memoryin accordance with an embodiment of the present invention having asingle charge storage medium and single control gate surrounding apillar.

[0044]FIG. 1C is an illustration of an overhead view showing a pillarmemory in accordance with an embodiment of the present invention havingmultiple charge storage mediums and multiple control gates.

[0045]FIG. 2 is an illustration of the pillar memory in accordance withan embodiment of the present invention.

[0046]FIGS. 3A-3D illustrate an ultra thin channel pillar memory devicein accordance with an embodiment of the present invention and its methodof fabrication.

[0047]FIG. 4 is an illustration of a pillar memory of an embodiment ofthe present invention having Schottky contacts.

[0048]FIG. 5 is an illustration of a gated diode pillar memory inaccordance with an embodiment of the present invention.

[0049]FIG. 6 is an illustration of a pillar memory in accordance with anembodiment of the present invention having a nanocrystal floating gate.

[0050]FIG. 7 is an illustration of a pillar memory of an embodiment ofthe present invention having a charge trapping dielectric.

[0051]FIGS. 8A and 8B illustrate a method of forming a pillar utilizingan explicit pillar formation process.

[0052]FIGS. 9A and 9B illustrate a method of forming a pillar utilizingan intersection etch technique.

[0053]FIGS. 10A-10E illustrate a method of forming a pillar memorydevice in accordance with an embodiment of the present inventionutilizing a “spacer etch” technique.

[0054]FIGS. 11A-11C illustrate a method of forming a common control gatebetween adjacent pillar memories as well as showing the isolation ofcontrol gates between adjacent pillars.

[0055]FIGS. 12A and 12B illustrate a method of forming a commoncontinuous film control gate between two or more levels of pillarmemories.

[0056]FIG. 13 to FIG. 28 illustrate a method of fabricating multiplelevels of pillar memories in accordance with an embodiment of thepresent invention.

[0057]FIG. 29A is a representation of a memory cell of an embodiment ofthe present invention.

[0058]FIG. 29B is a graph illustrating the characteristics of the cellof FIG. 29A.

[0059]FIG. 30 is a cross-sectional elevation view of a two terminal cellbuilt in accordance with an embodiment of the present invention.

[0060]FIG. 31 is a cross-sectional elevation view of a three terminalcell built in accordance with an embodiment of the present invention.

[0061]FIG. 32 is a cross-sectional elevation view of a three-dimensionalmemory array employing rail stacks built in accordance with anembodiment of the present invention.

[0062]FIG. 33 is a perspective view of a cell formed as a pillar above asubstrate in accordance with an embodiment of the present invention.

[0063]FIG. 34 is another embodiment of a cell formed as a pillar.

[0064]FIGS. 35 and 36 are schematics of a three dimensional array ofdevices.

[0065]FIG. 37 is a side cross-sectional view of a wafer after ONOdielectric, first gate electrode, protective oxide and blocking nitridelayers have been deposited in a method according to an embodiment of thepresent invention.

[0066]FIG. 38 is a side cross-sectional view of a memory array after bitline patterning and source/drain implantation. The cross-section isperpendicular to the bit lines.

[0067]FIG. 39 is a side cross-sectional view of the array after salicideprocess. The cross-section is perpendicular to the bit lines.

[0068]FIG. 40 is a side cross-sectional view of the array after theoxide fill and planarization. The cross-section is perpendicular to thebit lines.

[0069]FIG. 41 is a side cross-sectional view of the array after theblocking layer is removed. The cross section is perpendicular to the bitlines.

[0070]FIG. 42 is a side cross-sectional view of the array during wordline formation. The cross-section is perpendicular to the bit lines.

[0071]FIG. 43 is a side cross-sectional view of the array after wordline formation along line A-A in FIG. 42. The cross-section isperpendicular to the word lines and passes through a bit line.

[0072]FIG. 44 is a side cross-sectional view of the array after wordline formation along line B-B in FIG. 42. The cross-section isperpendicular to the word lines and passes through a transistor channel.

[0073]FIG. 45 is a side cross-sectional view of the array of the secondpreferred embodiment after the oxide fill and planarization. Thecross-section is perpendicular to the bit lines.

[0074]FIG. 46 is a side cross-sectional view of the array of the secondpreferred embodiment after word line formation. The cross-section isperpendicular to the bit lines.

[0075]FIG. 47 is a side cross-sectional view of the array of a preferredembodiment after word line formation. The cross-section is perpendicularto the bit lines.

[0076] FIGS. 48A-C and 49A-C illustrate alternative methods of making aTFT of the array of a preferred embodiment.

[0077]FIGS. 50 and 51 are side cross-sectional views of the array of twopreferred aspects of a preferred embodiment after word line formation.The cross-section is perpendicular to the bit lines.

[0078]FIG. 52 is a three dimensional view of a three dimensional arrayof a preferred embodiment.

[0079]FIG. 53 is a side cross-sectional view of a word line contactconductor and bit line contact conductor at the same level. Openings aremade for the next level contacts.

[0080]FIG. 54 is a side cross-section view of a word line contactconductor in level N+1 and word line and bit line contact conductors inlevel N. Landing pads are made in level N+1 conductor for the next levelcontacts.

[0081]FIGS. 55-61 are side cross-sectional views of a method of makingthe array of a preferred embodiment. The cross-section is perpendicularto the bit lines.

[0082]FIG. 62 is a top view of the array of a preferred embodiment ofthe present invention after forming crystallization windows.

[0083]FIGS. 63 and 64 are side cross-sectional views along lines A-A andB-B, respectively, in FIG. 62. The cross-section is perpendicular to thebit lines in FIG. 63 and parallel to the bit lines in FIG. 64.

[0084]FIG. 65 is a top view of the array of a preferred embodiment afterthe crystallization of the active layer.

[0085]FIG. 66 is a drawing showing a front perspective view of atwo-dimensional memory array in accordance with a specific embodiment ofthe present invention.

[0086]FIG. 67 is a drawing showing an elevational cross sectional viewof a two-dimensional memory array in accordance with a specificembodiment of the present invention.

[0087]FIG. 68 is a drawing showing a top plan view of a memory array inaccordance with a specific embodiment of the present invention.

[0088]FIG. 69 is a drawing showing an elevational cross sectional viewof a three-dimensional memory array in accordance with a specificembodiment of the present invention.

[0089]FIG. 70 is a drawing showing an elevational cross sectional viewof a two-dimensional memory array in accordance with a specificembodiment of the present invention.

[0090]FIG. 71 is a drawing showing an elevational cross sectional viewof a three-dimensional memory array in accordance with a specificembodiment of the present invention.

[0091]FIG. 72 is a drawing showing an elevational cross sectional viewof a memory array in accordance with a specific embodiment of thepresent invention.

[0092]FIG. 73 is a drawing showing an elevational cross sectional viewof a three-dimensional memory array in accordance with a specificembodiment of the present invention.

[0093]FIGS. 74 and 75 are drawings illustrating methods for programmingmemory cells in accordance with a specific embodiment of the presentinvention.

[0094]FIG. 76 is a drawing illustrating a method of fabrication ofmemory cells in accordance with a specific embodiment of the presentinvention.

[0095]FIG. 77 is a cross sectional drawing illustrating a SONOS on adielectric stack.

[0096]FIG. 78 is a cross-sectional drawing illustrating ananocrystalline charge storage medium.

[0097]FIG. 79 is a cross-sectional drawing of a bitline of dopedpolysilicon having a refractory metal silicide formed therein to improvelateral conductivity.

[0098]FIG. 80 is a cross-sectional drawing of a substrate in accordancewith a specific embodiment of the present invention.

[0099]FIGS. 81A-81H illustrate steps in the fabrication of a memoryarray in accordance with a specific embodiment of the present invention.

[0100]FIGS. 82A-82I illustrate steps in the fabrication of a memoryarray in accordance with a specific embodiment of the present invention.

[0101]FIGS. 83-85 illustrate flash memory arrays according to apreferred embodiment of the present invention.

[0102]FIGS. 86A-86J illustrate methods of making the arrays of FIGS.83-85.

[0103]FIG. 87 illustrates a CMOS array according to a preferredembodiment of the present invention.

[0104] FIGS. 88A-D illustrate a method of making the CMOS array of FIG.87.

[0105]FIGS. 89-92 illustrate logic and memory circuits using the CMOSarray of FIG. 87.

[0106]FIG. 93 is a process flow diagram illustrating a process forfabricating a crystallized amorphous silicon layer for use in anon-volatile TFT memory device in accordance with a specific embodimentof the present invention.

[0107]FIGS. 94A-94H are vertical cross-sectional drawings illustratingsteps in the process of FIG. 93.

[0108]FIG. 95 is a top plan view of a portion of a silicon wafer afterprocessing in accordance with the process of FIG. 93.

[0109]FIGS. 96-101 are illustrations of prior art devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0110] The present inventors have realized that the cost of memory andlogic devices would be decreased if the device density was increased.Thus, the present inventors have provided an ultra dense matrix array ofcharge storage semiconductor devices which has an increased density anda lower cost.

[0111] One method of improving device density is to arrange the devicesin a monolithic three dimensional array of charge storage devicescomprising a plurality of device levels. The term “monolithic” meansthat layers of each level of the array were directly deposited on thelayers of each underlying level of the array. In contrast, twodimensional arrays may be formed separately and then packaged togetherto form a non-monolithic memory device.

[0112] In order to form such a three dimensional array, especially anarray having four or more layers, at least one surface between twosuccessive device levels is planarized by chemical mechanical polishing(CMP). In contrast to other planarization methods, such as etch back,chemical mechanical polishing allows a sufficient degree ofplanarization to stack multiple device levels of a commercially feasibledevice on top of each other. The inventors have found that chemicalmechanical polishing typically achieves flatness on the order of 4000Angstroms or less within a stepper field (i.e., a peak to peak roughnessvalue of 4000 Angstroms or less in an area on the order of 10 to 50 mm)in three-dimensional memory arrays, even after 4 to 8 layers of thearray have been formed. Preferably, the peak to peak roughness of alayer in the array polished by CMP is 3000 Angstroms or less, such as500 to 1000 Angstroms, within a stepper field. In contrast, etch backalone typically does not afford sufficient flatness to achieve acommercially suitable three-dimensional memory or logic monolithicarray.

[0113] For example, the term “at least one surface between twosuccessive device levels is planarized by chemical mechanical polishing”includes surfaces formed in the bottom and intermediate device layers,as well as surfaces of the interlayer insulating layers that aredisposed in between the device layers. Thus, the surfaces of conductiveand/or insulating layers in each intermediate and bottom device level ofthe array are planarized by chemical mechanical polishing. Thus, if thearray includes at least four device levels, then at least three devicelevels should have at least one surface that is planarized by chemicalmechanical polishing. The surfaces of the conductive and/or insulatinglayers in the top device level may also be planarized by chemicalmechanical polishing.

[0114] Another method of improving device density is to verticallyintegrate the driver or peripheral circuits with the memory or logicarray. In the prior art, the peripheral circuits were formed in theperiphery of the monocrystalline silicon substrate, while the memory orlogic array was formed in the other portions of the substrate, adjacentto the peripheral circuits. Thus, the peripheral circuits occupiedvaluable substrate space in the prior art devices. In contrast, apreferred embodiment of the present invention provides a monolithicthree dimensional array of charge storage devices formed in an amorphousor polycrystalline semiconductor layer over a monocrystallinesemiconductor substrate, while at least part, and preferably all, thedriver (i.e., peripheral) circuitry is formed in the substrate under thearray, within the array or above the array. Preferably, the drivercircuitry comprises at least one of sense amps and charge pumps formedwholly or partially under the array in the substrate.

[0115]FIG. 35 schematically illustrates an array of charge storage logicor memory devices 3101 formed above an interlayer insulating layer 3102disposed above a monocrystalline substrate 3105. The array of chargestorage logic or memory devices 3101 are thus arranged as a threedimensional monolithic array thin film transistors or diodes inamorphous or polysilicon layers. The array 3101 has a plurality ofdevice levels 3104, preferably separated by interlayer insulatinglayers. The driver circuits 3103, such as sense amps and charge pumps,are disposed in the monocrystalline substrate 3105, as CMOS or othertransistors. FIG. 36 schematically illustrates an array of chargestorage logic or memory devices 3101 formed above a monocrystallinesubstrate 3105 as thin film transistors or diodes in amorphous orpolysilicon layers. The driver circuits 3103, such as sense amps andcharge pumps, are formed within the array 3101 and/or above the array3101.

[0116] Another method of improving device density is self-alignment andusing the same photolithography step to pattern different layers. Thedevice cell area is enlarged by misalignment tolerances that are putinto place to guarantee complete overlap between features on differentlayers. Thus, the present inventors have developed a fully or partiallyaligned memory cell structure that does not require misalignmenttolerances or that requires a reduced number of misalignment tolerances.In such a cell structure, certain device features may be self aligned toother device features, and do not require a photolithography step forpatterning. Alternatively, plural layers may be etched using the samephotoresist mask or a lower device layer may be etched using a patternedupper device layer as a mask. Particular examples of aligned memorycells will be discussed in more detail below.

[0117] The charge storage devices of the array may be any type ofsemiconductor devices which store charge, such as EPROMs or EEPROMs. Inthe preferred embodiments of the present invention described in detailbelow, the charge storage devices are formed in various configurations,such as a pillar TFT EEPROM, a pillar diode with a charge storageregion, a self aligned TFT EEPROM, a rail stack TFT EEPROM, and variousother configurations. Each of these configurations provides devices witha high degree of planarity and alignment or self-alignment to increasethe array density.

[0118] For example, in the pillar TFT EEPROM or a pillar diode with acharge storage region, at least one side of the semiconductor activeregion is aligned to one of the electrodes contacting the semiconductoractive region. Thus, in a pillar TFT EEPROM configuration, thesemiconductor active region is aligned to both the source and the drainelectrodes. This alignment occurs because at least two sides of theactive semiconductor region and one of the electrodes are patternedduring a same photolithography step (i.e., etched using the samephotoresist mask or one layer is used as a mask for the other layer).

[0119] In a self-aligned TFT, two sides of the active semiconductorregion are aligned to a side of the gate electrode only in the channelportion of the active semiconductor region, but not in the source anddrain regions. This alignment occurs because at least two sides of thechannel region and the gate electrode are patterned during a samephotolithography step (i.e., etched using the same photoresist mask orone layer is used as a mask for the other layer). In contrast, thesource and drain regions are not etched.

[0120] In the following description, numerous specific details are setforth such as specific thicknesses, materials etc. in order to provide athorough understanding of the present invention. It will be apparent toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-known concepts,circuit and fabrication techniques are not set forth in detail in ordernot to unnecessarily obscure the present invention.

[0121] Any feature of any embodiment described below may be used inanother embodiment. The first set of embodiments describes variouspillar devices, the second set of embodiments describes self-aligned TFTdevices and the third set of embodiments describes rail stack TFTdevices. The fourth and fifth set of embodiments describes how thesedevices may be used in a logic or memory circuit. The final set ofembodiments describes the use of metal induced crystallization toimprove the crystallinity of the device levels.

[0122] I. The Pillar Devices

[0123] The present embodiment is directed to thin film transistors(TFTs) and diodes arranged in a pillar configuration (i.e., the verticaldirection with respect to the substrate, where the length of the deviceis perpendicular to the substrate) and their method of fabrication.Preferably, the pillar devices form a charge trapping memory that has avertical read current. The memory includes a first input/outputconductor formed on or above a plane of a substrate and a secondinput/output conductor located above and spaced apart from the firstinput/output conductor. The first input/output conductor and the secondinput/output conductor are positioned so that they overlap or intersectone another and preferably intersect perpendicular to one another. Asemiconductor region, such as a doped silicon region, is formed betweenthe first input/output conductor and the second input/output conductorat the intersection of the first input/output conductor and the secondinput/output conductor. A charge storage medium, such as but not limitedto a charge trapping dielectric, is formed near the semiconductor regionand affects the amount of current that flows through the semiconductorregion between the first input/output conductor and the secondinput/output conductor for a given voltage applied across the firstinput/output conductor and the second input/output conductor. The amountof current (read current) for a single voltage that flows through thesemiconductor region can be used to determine whether or not charge isstored in the charge storage medium and therefore whether or not thememory is programmed or erased. The read current that flows through thesemiconductor region between the first input/output conductor and thesecond input/output conductor flows in a direction perpendicular to theplane of the substrate in which or on which the memory is formed. Thestructure of the charge trapping memory of the present embodiment, aswell as its method of fabrication, is ideally suited for integrationinto a three dimensional array of memory devices.

[0124] As will be discussed below, the charge trapping memory device ofthe present embodiment can be fabricated with one of two generalstructures. In one embodiment the charge storage medium is formedadjacent to the semiconductor region and in a second embodiment thecharge storage medium is formed above or below the semiconductor region.

[0125] 1. A Three Terminal Pillar Memory with Adjacent Charge StorageMedium

[0126] An embodiment of the present invention is a three terminalnonvolatile stackable pillar memory device. A pillar memory device 100in accordance with this embodiment of the present invention is broadlyillustrated in FIG. 1A. Pillar memory device 100 includes a firstcontact region 102 formed on a first input/output (I/O) 103 conductorformed on or above a plane (x-y) of a single crystal substrate 101. Asemiconductor body 104 is formed directly on the first contact region102 and a second contact region 106 is formed directly on the body 104.A second I/O conductor 116 is formed on the second contact region 106.The first contact region 102, the body 104, and the second contact(source/drain) region 106 are each vertically aligned with one anotherto form a pillar 108. Adjacent to and in contact with body 104 is acharge storage medium 110. A control gate 112 is formed adjacent to andin direct contact with the charge storage medium 110. The control gate112 and charge storage medium 110 are constructed so that they lielaterally adjacent to pillar 108 so that they may electricallycommunicate with pillar 108. The charge storage medium is the regionthat electrically screens the control gate and the channel regionaddressed by the control gate.

[0127] The programmed or unprogrammed state of the pillar memory deviceis determined by whether or not charge is stored in charge storagemedium 110. The charge stored in the charge storage medium adds orsubtracts from the voltage applied to the control gate, thereby alteringthe voltage required to form a conducting channel in body 104 to enablea current (e.g., read current IR) to flow between the first and secondcontact (source/drain) regions. This voltage is defined as the VT. Theamount of voltage required to form a conducting channel in body 104 orthe amount of current flowing in the body for a given control gatevoltage can be used to determine whether or not the device is programmedor unprogrammed. Additionally, multiple bits of data can be stored in asingle charge storage medium 110 whereby each different amount of storedcharge creates a different VT each representing a different state of thecharge storage medium. Because the charge storage medium can containmultiple states, multiple bits can be stored in a single charge storagemedium.

[0128] During read operations of device 100, when a conductive channelis formed in body 104, current 114 flows vertically (z) (orperpendicular) with respect to the plane (x-y) of the substrate 101above which pillar memory device is formed. By creating a memory devicewith a “vertical” read current path, the pillar memory cell of thepresent invention can be easily stacked in a three dimensional arraywith source/drain conductors 103 and 116 running parallel orperpendicular to each other and parallel to the plane of the substrate101 without requiring the use of vertical interconnect strategies forthe source and drain connections. The conductor 112 to the control gatemay be run vertically (as shown in FIG. 1A) or horizontally.

[0129] Although memory device 100 shown in FIG. 1A includes a chargestorage medium 110 and a control gate 112 formed on only one side orsurface of pillar 108, it is to be appreciated that the pillar memorydevice of the present invention can be fabricated so that the entirebody 110 of the pillar 108 is surrounded by a single charge storagemember 110 and a single control gate 112 as shown in FIG. 1B.Additionally, each surface of the pillar 108 can have an independentlycontrolled charge storage member and control gate as shown in FIG. 1Cand thereby enable multiple bits of data to be stored in a single pillarmemory device of the present invention. The use of multiple chargestorage members and control gates enables the storage of multiple valueson a single pillar device by determining how much of the channel isexposed to charge. Additionally, each face of body 104 of pillar 108 canhave different doping densities to create different threshold voltagesfor each face to further enable the pillar memory to store additionalstates and therefore additional bits.

[0130]FIG. 2 shows an embodiment of the present invention where thepillar 207 comprises a first source/drain contact region 202 comprisinga heavily doped N+ silicon film having a doping density in the rangebetween 1×10¹⁹ to 1×10²⁰, preferably 1×10¹⁹ to 1×10²¹ atoms/cm³, formedon a first input/output 204 (e.g. bit line) formed on or above asubstrate 201. A body comprising a lightly doped P− type silicon film206 having a doping density between 1×10¹⁶ to 1×10¹⁸ atoms/cm³ is formedon and in direct contact with the first N+ source/drain contact region202. A second source/drain region 208 comprising a heavily doped N+silicon film having a doping density of 1×10¹⁹ to 1×10²⁰, preferably1×10¹⁹ to 1×10²¹, atoms/cm³ is formed on and in direct contact with Ptype silicon film 206, as shown in FIG. 2. A second conductiveinput/output (e.g. word line/bit line) 210 is formed on the second N+source/drain region 208. The N+ source/drain films 202 and 208 can havea thickness between 500-1000 Å. The first and second input/outputs 204and 210 can be formed of a highly conductive material such as but notlimited to a metal such as tungsten, a silicide such as titaniumsilicide or tungsten silicide, or heavily doped silicon. In memorydevice 200 N+ source/drain region 202, P type silicon body 206 and N+source/drain region 208 are each substantially vertically aligned withone another to form pillar 207.

[0131] Pillar memory 200, shown in FIG. 2, has a charge storage medium211 comprising a tunnel dielectric 212, a floating gate 214, and acontrol gate dielectric 216. The tunnel dielectric is formed adjacent toand in direct contact with P type silicon body 206. A floating gate 214is formed adjacent to and in direct contact with tunnel dielectric 212.Floating gate 214 comprises a conductor such as but not limited to dopedsilicon, such as N type silicon, or metal such as tungsten. The controlgate dielectric 216 is formed adjacent to and in direct contact withfloating gate 214. Finally a control gate 218 is formed adjacent to andin direct contact with control gate dielectric 216. Control gate 218 isformed of a conductor such as but not limited to doped silicon or ametal such as tungsten.

[0132] The thicknesses of P type silicon film 206 and tunnel dielectric212 are dependent upon the desired programming and erasing voltage. Iflow voltage programming operations between 4 to 5 volts are desired,then P− type silicon film 206 can have a thickness between 1000-2500 Åand the tunnel dielectric can have a thickness between 20 and 150 Å,such as 20-50 Å, preferably 80-130 Å. (If a nitride tunnel dielectric212 is desired it would be scaled slightly thicker.) It is to beappreciated that the thickness of P− type silicon film 206 defines thechannel length of the device. If higher voltage (6-10 volts) programmingoperations are desired the P type silicon film 206 can have a thicknessbetween 6000-7000 Å and tunnel dielectric 212 can have a thicknessbetween 60-100 Å. The control dielectric 216 typically has a thicknesson order of tunnel dielectric 212 but is slightly (10-30 Å) thicker,preferably 130 to 180 Å.

[0133] Pillar memory 200 is considered programmed or unprogrammeddepending upon whether or not charge is stored on floating gate 214.Pillar memory device 200 can be programmed utilizing drain sideprogramming whereby electrons are placed on floating gate 214 bygrounding the source region 202 while a relatively high voltage isapplied to the drain region 208 and while approximately 4-5 volts, forlow voltage operations, or 6-10 volts, for high voltage operations, isapplied to control gate 218 in order to invert a portion of P− typesilicon region 206 into N type silicon so that a channel region isformed and electrons flow between the source region and the drainregion. The high control gate voltage pulls electrons from the invertedchannel region through the tunnel dielectric 212 and on to floating gate214. Because electrons lose some of their energy tunneling through thetunnel oxide, they no longer have enough energy to escape from thefloating gate which is surrounded by insulators. Other techniques suchas but not limited to source side injection can be used to programmemory device 200.

[0134] Memory device 200 can be erased by removing stored electrons fromfloating gate 214. Memory device 200 can be erased by placing arelatively high positive voltage (3 volts) on to the source region,while applying a negative voltage of approximately 4-5 volts in lowvoltage operations or 6-10 volts for high voltage operations on tocontrol gate 218. The positive voltage on the source region attractselectrons on floating gate 214 and thereby pulls electrons off floatinggate 214 through tunnel dielectric 212 and into the source region.

[0135] In order to read the state of memory device 200, a voltage (suchas 3.3 volts) can be applied to the drain while a given control gatevoltage is applied to the control gate. The amount of current (readcurrent) that flows from the drain region through the channel region andinto the source region for a given control gate voltage can be used todetermine the state of the memory device. Alternatively, one can readthe state of memory 200 by sensing the amount of control gate voltagenecessary to cause a given read current to flow through body 206. Whenread current flows between the first and second source/drain regions 202and 208 through body 206 it flows in a direction perpendicular (z) tothe plane (x-y) of the substrate 201 on or above which it is built.

[0136]FIG. 3 shows another embodiment of the nonvolatile pillar memorydevice of the present invention. FIG. 3 shows a three terminalnonvolatile pillar memory device 300 having an ultra thin siliconchannel or body 302. Like memory device 200 the ultra thin memory device300 has a first N+ source/drain contact region 202 formed on a firstinput/output 204. An insulator 304, such as an SiO₂ film or a siliconnitride film, is formed on the first source/drain contact region 202. Asecond N+ source/drain region 208 is formed on the insulating layer 304.Insulator 304 separates the source/drain regions 202 and 208 from oneanother and therefore defines the channel length of the device. A thinP− type silicon film 302 having a concentration in the range between1×10¹⁶ to 1×10¹⁸ atoms/cm³ is formed along the sidewalls of theN+/insulator/N+ stack so that it is adjacent to and in direct contactwith the first and second source/drain regions as well as separatinginsulator 304. The P− type silicon film acts as the channel or body forthe device and bridges the gap between source/drain regions 202 and 208.By forming a thin P− type silicon film adjacent to the N+/insulator/N+stack the channel region can be made extremely thin, between 50-100 Å.The thickness of the P− type silicon film which represents the channelthickness is preferably less than ½ the channel length (i.e. thedistance between the source/drain regions 202 and 208) and ideally lessthan ⅓ the channel length.

[0137] Like memory device 200, memory device 300 also includes a chargestorage medium 211, and a control gate 218. When transistor 300 isturned on, a portion of the P− type silicon region inverts to form aconductive channel therein so that current can flow from onesource/drain region 202 to the other source/drain region 208. Themajority of the current path 306 through the ultra thin body 302 orchannel from one source/drain region to the other source/drain region isin a direction perpendicular (z) to the plane (x-y) of the substrateabove which the device is built.

[0138] An ultra thin channel or body transistor can be formed, forexample, by using a “spacer etch” technique. For example, as shown inFIG. 3B an N+ silicon/insulator/N+ silicon stack can be blanketdeposited over a substrate having a patterned metal I/O 204. The stackis then patterned utilizing well-known photolithography and etchingtechniques into a pillar 306 is shown in FIG. 3B. A P− type silicon filmcan then be blanket deposited over the pillar as shown in FIG. 3C. TheP− type silicon film is deposited to a thickness desired for the channelthickness of the device. The P− type polysilicon film is thenanisotropically etched so that P− type silicon film 302 is removed fromhorizontal surfaces and remains on vertical surfaces such as thesidewalls of pillar 306. In this way the P− type silicon film is formedadjacent to the pillar and bridges the source/drain regions across theinsulator 304. The charge storage medium 211 and control gate 218 canthen subsequently be formed as in the other pillar devices.

[0139]FIG. 4 shows another embodiment of the three terminal stackablenonvolatile pillar memory device of the present invention. FIG. 4 is athree terminal stackable non-volatile pillar memory device whereSchottky contacts form the source and drain regions of the device. TheSchottky contact MOSFET 400 of the present invention includes a firstmetal contact 402 formed on a first input/output 204. A doped siliconbody or channel 404 such as N type silicon doped to a concentrationlevel between 1×10¹⁶ to 1×10¹⁸ atoms/cm³ and having a thickness desiredfor the channel length is formed on metal contact 402. A second metalcontact 406 is formed on and in direct contact with silicon body 404. Asecond I/O is then formed on second metal contact 406. First metalcontact 402 and second metal contact 406 are formed of a material suchas platinum silicide, tungsten silicide and titanium silicide and to athickness that forms a Schottky barrier contact with silicon body 404.The first metal contact 402, silicon body 404, and second metal contact406 are each directly vertically aligned to one another to form a pillar408 as shown in FIG. 4. Memory device 400 also includes a charge storagemedium 211 directly adjacent to and in contact with silicon body 404 asshown in FIG. 4. Additionally, memory device 400 includes a control gateadjacent to and in direct contact with the charge storage medium 211.When a channel is formed in silicon body 404, current (e.g., readcurrent I_(R)) flows from metal contact 402 to metal contact 406 in adirection perpendicular (z) to the surface of the substrate (x-y) onwhich memory device 400 is formed.

[0140]FIG. 5 illustrates another embodiment of a three terminalnonvolatile memory device in accordance with the embodiment of thepresent invention. FIG. 5 illustrates a gated diode memory device 500.Memory device 500 includes a P+ type silicon film contact region 502having a dopant density between 1×10¹⁹ to 10×10²¹, preferably 1×10¹⁹ to1×10²⁰ atoms/cm³ and a thickness between 500-1000 Å. A P− silicon film504 having a doping density between 1×10¹⁶ to 1×10¹⁸ atoms/cm³ is formedon and in direct contact with P+ silicon film 502. An N+ type siliconcontact region 506 having a doping density between 1×10^(19 to 10) ²¹,preferably 1×10¹⁹ to 1×10²⁰, atoms/cm³ and a thickness between 500-1000Å is formed directly on P− silicon film 504. In an embodiment of thepresent invention P+ silicon film 502, P− silicon film 504, and N+silicon film 506 are each vertically aligned with one another to form apillar 508 as shown in FIG. 5. Memory device 500 also includes a memorystorage medium 211 formed adjacent to and in direct contact with P−silicon film 504 and N+ silicon film 506 as shown in FIG. 5. Adjacent toand in direct contact with charge storage medium 211 is a control gate218. Additionally, like transistors 100, 200, 300, and 400, when gateddiode 500 is turned “on” a current (I) travels from P+ silicon film 502to N− type silicon film 506 in a direction perpendicular (z) to theplane (x-y) of the substrate 501 on or above which device 500 is formed.

[0141] Although devices 200-500 have been shown with a charge storagemedium comprising a continuous film floating gate 214 isolated by atunnel dielectric 212 and a control gate dielectric 216, the floatinggate need not necessarily be formed from a continuous conductive film ofsilicon or metal but can alternatively be formed from a plurality of aelectrically isolated nanocrystals 602 as shown in FIG. 6. Nanocrystalsare small clusters or crystals of a conductive material that areelectrically isolated from one another. An advantage of the use ofnanocrystals for the floating gate is that because they do not form acontinuous film, nanocrystal floating gates are self isolating.Nanocrystals 602 enable multiple self-isolating floating gates to beformed around a single silicon body 206. For example, with a square orrectangular shaped pillar, a floating gate can be formed on each side ofthe silicon body or channel enabling four or more isolated floatinggates to be formed around a single square pillar. In this way, multiplebits can be stored in each pillar memory. Similarly, becausenanocrystals form a non-continuous film, floating gates can be formedafter two or more levels of pillars are formed without worrying aboutshorting of the floating gate of one cell level to the floating gates toadjacent cells lying directly above or below (i.e., verticallyadjacent). Yet another advantage of the use of nanocrystals for floatinggates is that they experience less charge leakage than do continuousfilm floating gates.

[0142] Nanocrystals 602 can be formed from conductive material such assilicon, tungsten, or aluminum. In order to be self isolating, thenanocrystals must have a material cluster size less than one-half thepitch of the cell so that floating gates from vertically andhorizontally adjacent cells are isolated. That is, the nanocrystals ormaterial clusters 602 must be small enough so that a single nanocrystal602 cannot bridge vertically or horizontally adjacent cells. Siliconnanocrystals can be formed from silicon by utilizing chemical vapordeposition to decompose a silicon source gas such as silane at very lowpressure. Similarly, a tungsten nanocrystal floating gate can be formedby chemical vapor deposition by decomposing a tungsten source gas suchas WF₆ at very low pressures. Still further, an aluminum nanocrystalfloating gate can be formed by sputter deposition at or near the meltingtemperature of aluminum.

[0143] Additionally, alternative to the use of a dielectric isolatedfloating gate to store charge in the memory devices of the presentinvention, one can use a trapping layer formed in the dielectric stack702 as shown in FIG. 7. For example, the charge storage medium can be adielectric stack 702 comprising a first oxide layer 704 adjacent to thesilicon body or channel, a nitride layer 706 adjacent to the first oxidelayer and a second oxide layer 708 adjacent to the nitride layer andadjacent to the control gate 218. Such a dielectric stack 702 issometimes referred to as an ONO stack (i.e., oxide-nitride-oxide) stack.Other suitable charge trapping dielectric films such as an H+ containingoxide film can be used if desired.

[0144] It is to be appreciated that each of the memory devices 200-500shown in FIGS. 2-5 can be made of opposite polarity by simply reversingthe conductivity type of each of the silicon regions in the pillar andmaintaining concentration ranges. In this way, not only can NMOS devicesbe fabricated as shown in FIGS. 2-5, but also PMOS devices can be formedif desired. Additionally, the silicon films used to form the pillars ofthe device may be single crystal silicon or polycrystalline silicon.Additionally, the silicon film can be a silicon alloy film such as asilicon germanium film doped with N type or P type conductivity ions tothe desired concentration.

[0145] Additionally, as shown in FIGS. 1-3 and 5, the pillars 108, 208,308, and 508 are fabricated so that the contacts and body are alignedwith one another when viewed from the top. This may be achieved by firstforming an I/O 204 and then blanket depositing the pillar film stack(e.g., N+/P−/N+) as shown in FIG. 8A. The film stack 802 can then bemasked and all three films anisotropically etched in a single step asshown in FIG. 8B to form a pillar 804. An explicit pillar formation stepcan form a pillar having any desired shape. For example, the pillar 804can take the shape of a square as shown in FIG. 8B or can take the shapeof rectangle, or a circle when viewed from above.

[0146] Alternatively, as shown in FIGS. 9A and 9B, a pillar can beformed by the intersection of the patterning of the first and secondI/O's. For example, a pillar can be formed by first blanket depositing afirst I/O conductor 900 followed by the sequential blanket deposition ofthe film stack 902 (e.g., N+/P−/N+) of the desired pillar. The first I/Ofilm 900 and the pillar film stack 902 are then etched to form aplurality of pillar strips 904 as shown in FIG. 9a. During subsequentprocessing to pattern the second I/O, the second I/O 906 is etched in adirection perpendicular or orthogonal to the plurality of strips 904.The etch step used to pattern the second I/O 906 is continued so as toetch away the pillar film stack 902 from the portions of the strip 904which are not covered or masked by the second I/O 906. In this way, apillar 908 is formed at the intersection of the first and second I/O's.The pillar 908 is formed in direct alignment with the intersection oroverlap of the first and second I/O's. The intersection technique offorming a pillar is advantageous because it saves additional lithographysteps.

[0147] The charge storage medium of the memory device of the presentinvention can be formed utilizing a “spacer etch” technique. Forexample, as shown in FIG. 10A-10E a pillar 1000 or a pillar strip isfirst formed. A first tunnel dielectric 1002 is then blanket depositedover the pillar 1000. Next, a floating gate material 1004 is blanketdeposited over the tunnel dielectric 1002. The floating gate dielectricmaterial is deposited to a thickness desired for the floating gate. Thefloating gate material can be nanocrystals or can be a continuousconductive film. The floating gate material 1004 and the tunneldielectric 1002 are then anisotropically etched back to remove them fromhorizontal surfaces such as the top of pillar 1000 and between adjacentpillars so as to leave a floating gate 1008 isolated by a tunneldielectric on the sidewalls of the pillar 1000 or strip. If the floatinggate is made from a continuous conductive film, as opposed tonanocrystals, then care must be taken to ensure the complete removal ofthe floating gate material 1004 from between adjacent cells so that thefloating gates 1008 of adjacent cells are isolated.

[0148] It is to be appreciated that when the floating gate is made ofnanocrytals or when the charge storage medium is a trapping dielectric,the films need not necessarily be etched from horizontal surfacesbetween adjacent cells because these films do not electrically coupleadjacent cells. If desired, however, charge trapping dielectric andnanocrystal floating gates can be anisotropically etched back. Next, asshown in FIG. 10D, a control gate dielectric 1006 is blanket depositedover floating gate 1008 and the top of pillar 1000.

[0149] A control gate can also be formed using a “spacer etch”technique. In such a case, a control gate material 1010, such as dopedpolysilicon, is blanket deposited over the control gate dielectric 1006to the thickness desired of the control gate as shown in FIG. 10D. Thecontrol gate material 1010 is then anisotropically etched back as shownin FIG. 10E to remove the control gate material 1010 from horizontalsurfaces such as on top of control gate dielectric 1006 and betweenadjacent pillars or strips and form a control gate 1012 adjacent tocontrol gate dielectric 1006. The control gate dielectric 1006 protectsthe underlying silicon pillar 1000 from being etched during theanisotropic etch of the control gate material.

[0150] While it is necessary to isolate the floating gate from adjacentcells, the control gate can be shared between horizontal or verticallyadjacent cells. Horizontally shared control gates can be achieved byutilizing lithography to form a conductor strip which connectshorizontally adjacent transistors. Alternatively, as shown in FIGS.11A-11C, horizontal coupling of adjacent cells can be achieved byaccurately controlling the space between adjacent cells 1100 so that aminimal space 1102 is placed between cells having control gates to becoupled together while larger gaps 1104 are placed between cells havingcontrols gates which are to be isolated as shown in FIG. 11A. In thisway, when a control gate material 1106 is deposited, it completely fillsthe minimum or small gaps 1102 between adjacent cells while leaving onlya thin film on the large gaps 1104 between cells to be isolated as shownin FIG. 11B. During the anisotropic etch, the thin control gate materialin the large gaps is completely removed, isolating adjacent controlgates, while a portion 1108 of the thicker control gate material 1106 inthe small gap remains, so that it bridges adjacent cells and coupleshorizontally adjacent cells as shown in FIG. 11C.

[0151] Additionally, vertical sharing of the control gate can beachieved by forming a control gate plug between adjacent cells after twoor more levels of pillar have been formed as shown in FIGS. 12A and 12B.A control gate plug can be formed by blanket depositing a conductivefilm such as a doped polysilicon film or a tungsten film 1200 over andbetween two or more levels of pillars and then planarizing or patterningthe portion of the tungsten film above the pillars to form a plugbetween pillars. In this way, the control gate would be shared withdevices on two or more vertical levels and between horizontally adjacentcells.

[0152] A method of integrating the pillar memory device of the presentinvention into a multi-level array of storage cells will now bedescribed. As shown in FIG. 13, the fabrication starts by providing asubstrate 1300 on which the multilevel array of storage devices is to beformed. Substrate 1300 will typically include a lightly dopedmonocrystalline silicon substrate 1302 in which transistors such asmetal oxide semiconductor (MOS) transistors are formed. Thesetransistors can be used as, for example, access transistors or they canbe coupled together into circuits to form, for example, charge pumps orsense amps for the fabricated memory devices. Substrate 1300 willtypically also include multiple levels of interconnects and interlayerdielectrics 1304 used to couple transistors in substrate 1302 togetherinto functional circuits. The top surface 1306 of substrate 1300 willtypically include an insulating layer or passivation layer to protectthe underlying transistors and interconnects from contamination. The topsurface 1306 will typically contain electrical contact pads to whichmultilevel arrays of memory devices of the present invention can beelectrically coupled in order to make electrical contact with thetransistors in silicon substrate 1302. In an embodiment of the presentinvention, the memory devices are physically isolated and separated fromthe single crystalline substrate by multiple levels of interconnects anddielectric 1304. The top surface of passivation or insulating layer 1306will typically be planarized to enable uniform and reliable fabricationof multiple levels of the charge storage devices of the presentinvention. FIG. 13A shows a cross-sectional view through the substratewhile FIG. 13B illustrates an overhead view of the substrate lookingdown at the plane of the substrate 1300 across which the devices of thepresent invention are fabricated. According to one embodiment of thepresent invention, the memory devices are physically separated frommonocrystalline silicon substrate 1302. In an alternative embodiment ofthe present invention, memory devices can be fabricated on a glasssubstrate 1300 such as used in flat panel displays.

[0153] A process of forming a multilevel array of memory devices inaccordance with an embodiment of the present invention begins by blanketdepositing a first conductor layer 1308 over surface 1306 of substrate1300. Conductor 1308 can be any suitable conductor such as but notlimited to, titanium silicide, doped polysilicon, or a metal such asaluminum or tungsten and their alloys formed by any suitable technique.Conductor layer 1308 is to be used as, for example, a bitline or awordline to couple a row or column of memory devices together. Next, astack 1310 of films from which the first level of pillars is to befabricated is blanket deposited over conductor 1308 as shown in FIG.13A. For example, in one embodiment the pillar is to comprise an N+source/drain region, a P− silicon body, and an N+ silicon source/drainregion. A suitable film stack 1310 can be formed by first blanketdepositing an amorphous silicon film by chemical vapor deposition (CVD)which is in situ doped with N type impurities to a doping densitybetween 1×10¹⁹ to 1×10²¹, preferably 1×10¹⁹ to 1×10²⁰, atoms/cm³. Next,a P− silicon film is deposited over the N+ silicon film 1312, by forexample, depositing an amorphous silicon film by chemical vapordeposition and which is in situ doped with P type impurities (e.g.,boron) to a dopant density of between 1×10¹⁶ to 1×10¹⁸ atoms/cm³. An N+silicon film 1316 is then blanket deposited over P− silicon body 1314 bydepositing a amorphous silicon film by chemical vapor deposition and insitu doping it to a level between 1×10¹⁹ to 10×10²¹, preferably 10×10¹⁹to 1×10²⁰, atoms/cm³. The amorphous silicon films can then be convertedinto polycrystalline silicon through a subsequent anneal. Alternative toin situ doping, the stack of films can be deposited as undoped siliconand then implanted or diffused with dopants.

[0154] It is to be appreciated that other memory devices in accordancewith the present invention can be fabricated by depositing appropriatefilm stacks to achieve their pillar configurations such asmetal/silicon/metal strip to form a device 400 as shown in FIG. 4, aP+/P−/N+ stack to form a device 500 as shown in FIG. 5, as well as anN+/SiO₂/N+ stack to form a device 300 as shown in FIG. 3A. Next, asshown in FIGS. 14A and 14B the blanket deposited film stack 1310 andmetal conductor 1308 are patterned utilizing well-known photolithographyand etching techniques to form a plurality of pillar strips 1318. Thefilms of the deposited film stack 1310 and metal conductor 1308 areetched in alignment with one another and form strips with verticalsidewalls.

[0155] Next, as shown in FIGS. 15A and 15B, if desired, the substratecan be subjected to threshold adjusting ion implantation steps in orderto alter the doping density of the surface or face of the P type siliconregion on each strip. That is, at this time, a first ion implantationstep 1315 can be used to implant one surface of pillar 1318 with P typedopants to increase its P type doping density or can be implanted with Ntype dopants to counterdope and decrease its P type doping density.Similarly, after the first implant 1315, the substrate can be rotatedand subjected to a second ion implantation step 1317 to alter the dopingdensity of the opposite side or face of pillars strips 1318. Thethreshold adjustment implants should be of a sufficient dose tosufficiently alter the threshold voltage of each face so as to be ableto sufficiently distinguish or sense different read currents associatedwith each face. The angle of the ion implantation step is chosen so thatthe bulk of the implantation occurs into the surface of the P type body1314. The angle of the implant is dependent upon the strip height aswell as on the spacing between strips 1314.

[0156] Next, as shown in FIGS. 16A and 16B, tunnel dielectric 1320 isformed over the sidewalls and the top of strip 1318 as well as onsubstrate 1300 between strips 1318. Tunnel dielectric can be an oxide, anitride, a oxynitride, or other suitable dielectric. The tunneldielectric 1320 is preferably deposited utilizing a plasma deposition orgrowth process at a temperature of less than 750° C. and preferably lessthan 600° C. The tunnel dielectric 1320 is formed to a thickness andquality to prevent breakdown and leakage at operating conditions. Next,as also shown in FIGS. 16A and 16B, a floating gate material 1322 isblanket deposited over tunnel dielectric 1320. In a preferred embodimentof the present invention, the floating gate material is formed ofnanocrystals.

[0157] Silicon nanocrystals can be formed by depositing silicon in amanner whereby silicon has a very high surface diffusivity relative toits sticking coefficient. For example, silicon nanocrystals can beformed by chemical vapor deposition (CVD), by decomposing silane (SiH₄)at a very low pressure, between 1 millitorr to 200 millitorr, at atemperature between 250-650° C. In such a process, a very thindeposition, between 50-250 Å, will form little islands 1322 of silicon.If H₂ is included with silane during the deposition, higher pressurescan be utilized and still obtain nanocrystals. In an alternativeembodiment of the present invention, metal nanocrystals such as aluminumnanocrystals, can be formed by sputtering from a metal target at atemperature near the melting temperature of the metal, so that the metalagglomerates and forms nanocrystals. Tungsten nanocrystals can be formedby chemical vapor deposition utilizing a reactant gas mix comprising atungsten source gas such as WF₆ and germane (GeH₄). In still yet anotherembodiment of the present invention, a continuous film of floating gatematerial can be deposited and then caused to precipitate (by heating) tocause islands to form in the film.

[0158] It is to be appreciated that although nanocrystals are preferredfor the floating gate because of their self isolating quality, thefloating gate can be formed from a continuous film such as, but notlimited to, a metal such as tungsten or a silicon film such aspolycrystalline or amorphous silicon doped to the desired conductivitytype (typically N+ silicon for an N+/P−/N+ pillar). If a continuous filmis used as floating gate material 1322, the film 1322 would beanisotropically etched at this time to remove the portion of thefloating gate material 1322 between strips 1318 to electrically isolatethe strips.

[0159] Next, as also shown in FIGS. 16A and 16B, a control gatedielectric 1324 is blanket deposited over and onto floating gatematerial or nanocrystals 1322. The control gate dielectric 1324 is adeposited dielectric of, for example, an oxide or oxynitride film formedby a plasma enhanced deposition process to reduce the depositiontemperature. The control gate dielectric 1324 has a thickness similar tothe tunnel dielectric 1320 but slightly, e.g., 20-30 Å, thicker. Thecontrol gate dielectric 1324 is used to isolate the floating gate from asubsequently formed control gate. The thickness and quality of thecontrol gate dielectric depends upon the program threshold voltage forprogramming and unprogramming the memory cell. As discussed above, thethickness of the tunnel dielectric as well as the thickness of the Ptype silicon body or channel are dependent upon the programming voltagedesired.

[0160] Next, as shown in FIGS. 17A and 17B, a control gate material 1328is blanket deposited on and over strips 1318. The control gate materialis formed to a thickness at least sufficient to fill the gaps betweenadjacent strips. Typically, a conformal film deposited to a thickness ofat least one-half the width of the gap 1330 will ensure complete fillingof gap 1330. In an embodiment of the present invention, the control gatematerial 1328 is a doped polycrystalline silicon film formed by chemicalvapor deposition. Alternatively, the control gate can be formed fromother conductors such as a blanket deposited tungsten film formed bychemical vapor deposition utilizing WF₆. Next, as shown in FIGS. 18A and18B, the control gate film 1328 is planarized back by for example,chemical mechanical polishing until the top surface of the control gateis substantially planar with the control gate dielectric on the top ofstrips 1318. A plasma etch process is then utilized to recess 1331 thetop surface of the control gate material below the top surface of strips1318 and preferably to slightly above the top source/body junction(e.g., junction of N+ silicon film 1316 and P− silicon film 1314) asshown in FIG. 18A. The control gate dielectric 1324 on the top of strips1318 protects strips 1318 from etching during the recess etch. After therecess etch, control gates 1332A and B have been formed.

[0161] Next, an interlayer dielectric (ILD) 1334 such as an oxide, isblanket deposited over the top of strips 1318 as well as on and intorecesses 1331 over control gate 1332. The deposited oxide layer 1334, aswell as the control gate dielectric, the nanocrystals, and tunneldielectric on the top of strips 1318 are then polished or etched back asshown in FIGS. 19A and 19B to reveal and open the surface of the topsource/drain region (e.g., N+ film 1316) of each pillar strip 1318.

[0162] Next, as shown in FIGS. 20A and 20B, a second conductor layer1336 is blanket deposited over and in contact with the top source/drainregion (N+ source/drain region 1316) as well as over and onto ILD 1334.The second conductive layer 1336 will be used to form a secondinput/output (e.g., a bitline or a wordline) for the first level ofmemory devices and will be used to form a first input/output (e.g., awordline or a bitline) for the second level of memory devices. Secondconductive layer 1336 can be formed of materials and to thicknessessimilar to first conductive layer 1308.

[0163] Next, a film stack 1338, such as an N+/P−/N+ stack, used to formthe second level of pillars, is blanket deposited over second conductivelayer 1336 as shown in FIGS. 20A and 20B. The film stack 1338 can beformed with the same materials and to the same thickness as used forfilm stack 1310. Alternatively, if a different type of memory device isdesired, then a film stack corresponding to that device type would beformed.

[0164] Next, as illustrated in FIGS. 21A and 21B, the second pillarstack 1338 and the second conductive layer 1336 are patterned withwell-known photolithography and etching techniques to form a pluralityof second pillar strips 1340 orthogonal or perpendicular to the firstplurality of pillar strips 1318. It is to be appreciated that the filmsof the second pillar stack 1338 and the second conductive layer 1336 areetched in alignment with one another to form a strip with substantiallyvertical sidewalls.

[0165]FIGS. 22A and 22B show the substrate of FIGS. 21A and 21B rotated90°.

[0166] Once the second pillar film stack 1338 and second conductor 1336have been patterned by etching into a strip 1340, the etch is continuedto remove the portion 1341 of the first pillar strips 1318 not coveredor masked by the second pillar strips 1340 as shown in FIGS. 23A and23B. The etch is continued until the first conductive layer 1308 isreached. In this way, as shown in FIGS. 23A and 23B, a first level ofsquare or rectangular pillars 1342 have been formed from first pillarstrips 1318 at the intersections or overlaps of the first and second I/O1308 and 1336 (shown as M1 and M2 in FIG. 23A). In an embodiment of thepresent invention square pillars having a width of less than 0.18 μm areformed. It is to be appreciated that the etch step preferably uses anetch that can selectively etch the pillar strip with respect to the ILD1334 and the tunnel and control gate dielectrics. For example, if thepillar comprises doped silicon and the ILD and the tunnel and controlgate dielectrics are oxides, then a plasma etch utilizing Cl₂ and HBrcan etch silicon without significantly etching the oxide ILD or tunneland control gate dielectrics. It is to be appreciated that ILD 1334protects the underlying silicon control gate 1332 from being etched asshown in FIG. 23C. Additionally, the purpose of ILD 1334 is toelectrically isolate control gates 1332 from subsequently formed controlgates for the second level of pillars.

[0167] At this time, if desired, the substrate can be subjected tosuccessive ion implantation steps to alter the doping density of eachnewly revealed surface of P type body 1314 of pillar 1342 (see FIG. 23A)in order to alter the doping density of each face and therefore thethreshold voltage of each face.

[0168] Next, as shown in FIG. 24, a tunnel dielectric 1344, ananocrystal floating gate material 1346, and a control gate dielectric1348 are each successively blanket deposited over substrate 1300 to forma tunnel dielectric/floating gate/control gate on the sidewalls ofpillar devices 1342 as well as along the sidewalls of the second pillarstrip 1340 (see FIG. 23A). This film stack also forms along the topsurface of the second pillar strips 1340 as well as on the firstconductor 1308 between the first level of pillars 1342 and on ILD 1334.

[0169] The floating gate material need not be anisotropically etched toremove floating gate material from gaps 1343 between adjacent pillars1342 in order to isolate the pillars because although the floating gatematerial is conductive the non-continuous nature of the nanocrystalsprovides isolation between the pillars. In this way, the tunneldielectric, floating gate, and control gate dielectric can be used toisolate a subsequently formed control gate from the first metalconductor. Additionally, because the floating gate 1346 is formed fromnanocrystals, it is self isolating from the floating gate positioneddirectly above in Level 2 even though they have been formed at the sametime.

[0170] Next, as shown in FIG. 25A a control gate 1350 is formed betweensecond pillar strip 1340 as well as in the gaps 1343 between pillars1342. The control gate can be formed as discussed above with respect toFIGS. 17-20 whereby a control gate film, such as doped polysilicon, isblanket deposited to fill the gaps 1343 between adjacent pillars 1342 aswell as the gaps between second pillar strips 1340. Optionally, thecontrol gate film would then be polished and recessed back below the topsurface of the N+ source/drain regions and a second ILD 1352 formed inthe recesses as shown in FIG. 25A to allow additional layers to beadded. ILD 1352, the tunnel dielectric/floating gate/control gatedielectric on the top of the second pillar strip 1340 would then bepolished back to reveal the top N+ source/drain regions of strips 1340.

[0171] At this point, the fabrication of the first level of memorydevices is complete. Each pillar 1342 on the first level includes aseparate floating gate and control gate on each face of the pillar for atotal of four independently controllable charge storage regions as shownin FIG. 26. That is, as illustrated in FIG. 26, pillar 1342 contains afirst pair of control gates 1332A and B formed along laterally oppositesidewalls of the pillar 1342. The control gates 1332A and B are eachalso shared with the horizontally adjacent pillars. Pillar 1342 alsocontains a second pair of control gates 1350 Å and B formed alonglaterally opposite third and fourth faces of pillar 1342. Each controlgate 1350 will be shared with the subsequently formed pillar memorydevice position vertically above, in Level 2, as well as withhorizontally adjacent pillars 1342 in the same level. Because pillar1342 contains four independently controllable control gate and fourassociated and isolated floating gates, each pillar memory device 1342is able to store multiple states.

[0172] The process as described with respect to FIGS. 20-25 can berepeated again to complete the fabrication of memory devices on thesecond level and to begin the fabrication of the memory device on thethird level. That is, as shown in FIGS. 27A and 27B (FIG. 26 rotated90°) the steps of FIGS. 20-25 can be repeated to form third pillarstrips 1360 orthogonal to the second pillar strips 1340 which are usedto pattern the second pillar strips 1340 into a plurality of secondpillars 1362 on a second level and to form a second pair of controlgates 1364 adjacent to the second pillars.

[0173] In this way, a second level of memory pillars 1362 are fabricatedwhich contain four independently controllable control gates and fourassociated and isolated floating gates. A first pair of control gates1350A and B are formed along laterally opposite sidewalls of the secondlevel of pillars 1362 and are shared with memory pillar 1342 located onthe first level as well as with horizontally adjacent cells. A secondpair of control gates 1364A and B are formed along the third and fourthlaterally opposite faces of the second level of pillars 1362 and areshared with the subsequently formed pillars in the third level of thememory array.

[0174] The above described processes can be repeated as many times asdesired to add additional levels of pillar memory to the array. Thefinal level of memory cells can be patterned from a pillar stack stripwhile patterning the final I/O.

[0175] Although the three terminal memory pillar devices of the presentinvention have been shown integrated into a three dimensional memoryarray in a specific preferred embodiment, it is to be appreciated thatother methods may be utilized to fabricate a three dimensional memoryarray without departing from the scope of the present invention.

[0176] 2. Memory Cells Utilizinq a Charge Storage Medium Located Aboveor Below a Semiconductor Region

[0177] In FIG. 29A, the cell comprises a diode and a stack comprisingregions 2921, 2922 and 2923. The region 2921 comprises a firstdielectric region and the region 2923 comprises a second dielectricregion. Disposed between these regions is a storage region 2922 which isused to trap charge. It is primarily this region that retains charge andthus provides the “memory” of the cell. As will be described below,charge can be electrically placed within the region 2922, electricallysensed and electrically removed from the region 2922.

[0178] The region 2921 comprises an oxide with a thickness, typicallybetween 1-5 nm, and preferably 2-3 nm. In one embodiment, the region2921 is referred to in this application as a tunnel dielectric. Theregion 2922 is a region that stores trapped charge, as known in theprior art such as a nitride region (discussed in more detail below). Inone embodiment, the region 2922 is referred to in this application as astorage dielectric. The region 2923, which may comprise an oxide, actsas a barrier for retaining a trapped charge and in one embodiment isreferred to in this application as a blocking dielectric. It may havethicknesses similar to those of region 2921.

[0179] Because electrons carry the forward current in the diode oncepunch through occurs, these are the species that are trapped at thetunnel dielectric-storage dielectric interface 2925 and within theregion 2922. Note that these electrons are of a polarity to encouragethe premature inversion of the N region at the interface region 2921.Thus, stored electrons reduce the voltage at which first appears thenegative-resistance portion of the cell's characteristic, see curve 2926versus curve 2927 of FIG. 29B.

[0180] In one embodiment, programming consists of applying a sufficientforward bias to the diode to cause the device to conduct and allowingforward current to persist long enough for sufficient charge to becometrapped thereby shifting the voltage threshold from the peak forwardvoltage shown for curve 2927 to the peak forward voltage shown for curve2926. While throughout the discussion that follows, binary programmingis discussed, multiple bits may be stored per cell by employing multiplevalues of threshold shifts. By analogy, some flash memories store 2-4bits per cell or even more.

[0181] Reading (sensing) may be performed by applying a forward voltagethat falls between the peaks 2928 and 2929. If current in excess of apredetermined threshold value flows, the cell is programmed; ifconduction does not occur it is not programmed. The conduction that doesflow through a programmed cell during a read operation reinforces thetrapped charge.

[0182] Erasing is accomplished by applying a sufficient reverse bias tothe memory cell that electrons tunnel out of the traps, through theblocking oxide 2923 or through the flow of holes so as to neutralize thetrapped electrons. This action necessarily requires the diode to operatein breakdown, so the erase voltage will require at least the lower endof a breakdown voltage.

A. Two Terminal Cell in a Substrate

[0183] Referring to FIG. 30, a first embodiment of the invented memorycell is illustrated disposed in a p− type substrate 2930. A diode(steering element of the cell) is formed in the substrate comprising ann− region 2932, doped, for instance to a level of 5×10¹⁶-10¹⁸ cm⁻³, anda p+ region 2931, doped to >10¹⁹ cm⁻³ formed within the n− region 2932.These regions may be formed with well-known methods such as diffusion orion implantation.

[0184] A storage stack comprising a dielectric (e.g., oxide) region2933, trapping layer 2934 and a second dielectric (e.g., oxide) region2935 is formed on the region 2932.

[0185] The dielectric region 2933 may be a grown oxide layer or adeposited silicon dioxide region. When comprising oxide, this region maybe 1-5 nm thick. Ordinary processing may be used to form these regions.

[0186] The trapping region 2934 and the other trapping regions discussedin this application may be formed from a compound of nitrogen as well asother materials. In the prior art, silicon nitride (nitride) was mostcommonly used for this purpose. Other layers that may be used that havecompounds of nitrogen are oxynitride (ON) and oxide-nitride-oxide (ONO).Other materials, alone or in combination, that exhibit charge trappingcharacteristics can be used. For instance, alumina (Al₂O₃) and silicondioxide with insulated regions of polysilicon exhibit thesecharacteristics. The trapping region is generally between 2-20 nm thick,and preferably 3-10 nm thick.

[0187] The regions 2933 and 2934 have thicknesses determined by factorswell-known in the art for SONOS memories. For example, the tunneldielectric region needs to be thin enough to permit tunneling withoutexcess voltage drop and to provide longevity, while the trappingdielectric region must be thick enough not to allow significantspontaneous detrapping of charge. As mentioned above, typicalthicknesses are in the range of 1-5 nm, and preferably 2-3 nm for theoxide region 2933 and 3-10 nm for the trapping region where nitride isused.

[0188] The layer 2935 is an oxide or other dielectric region which mayhave the same thickness as region 2933. Other dielectrics that may beused include perovskites, ceramics, diamond (and diamond-like films),silicon carbide, and undoped silicon (including polysilicon). Thisregion may be formed by well-known deposition techniques. The region2933, as previously mentioned, is referred to as a tunnel dielectriclayer and is responsible, at least in part, for the negative-resistancecharacteristics previously discussed. The layer 2935, on the other hand,prevents trapped charge from region 2934 from leaking to, for instance,contact 2938. Hence, layer 2935 is sometimes referred to as the blockingdielectric.

[0189] The storage stack comprising regions 2933, 2934 and 2935 may befabricated in a single, continuous process where, for instance, gasmixtures in a deposition chamber are altered to first provide oxide thennitride and finally oxide again. Because of the relative thinness ofthese regions, the entire stack may be laid down in a matter of seconds.

[0190] To operate the cell of FIG. 30 first assume that uponmanufacturing the trapping layer is neutral, that is, there is notrapped charge in the trapping region 2934. To place charge in theregion 2934 the anode contact 2937 is brought to a positive potentialrelative to the contact 2938 in order to forward bias the diode definedby the regions 2931 and 2932 until the potential reaches the voltage2929 shown in FIG. 29B. Now tunneling occurs through the oxide 2933 aswell as the oxide 2935 and charge is trapped within the region 2934. Theamount of charge trapped depends on total current flow and the trappingefficiency of the region 2934.

[0191] To sense the presence of this charge, a potential is appliedbetween lines 2937 and 2938 again to forward bias the diode defined byregions 2931 and 2932. However, this time the potential is in a rangegreater than the voltage 2928 shown in FIG. 29B but less than thevoltage 2929. If current in excess of a predetermined threshold flows,then it is known that charge is trapped in the region 2934. On the otherhand, if such current flow does not occur, it is known that little or nocharge has been stored in the layer. In this way it can be determinedwhether the cell is programmed or not programmed for the binary datacase. As previously mentioned, different levels of charge may be placedin the trapping layer 2934, and the voltage at which said current flowoccurs (say between voltages 2928 and 2929) can be determined. Thiscorresponds to the amount of charge in the layer 2934 that can be usedto provide more than one bit of data from an individual cell.

[0192] It should be noted that during a read operation the read currentpasses through a programmed cell, and then passes through the region2933, trapping region 2934 and the oxide region 2938. This is unlike thetypical sensing that occurs where trapped charge is used to shift athreshold voltage in, for example, a field-effect transistor where thecurrent does not pass through the trapped charge region itself whenreading the state of the cell. As mentioned earlier, when the currentdoes pass through the region 2934 for reading it, in effect, refreshesthe cell; that is if the cell was originally programmed it will remainprogrammed when the data is read from the cell.

[0193] Care must be taken when reading data from the cell not to exceeda current represented by line 2924. If a current exceeds this limit, forexample, 5000-10,000 amps/cm², one or both of the oxide regions 2933 or2935 may be permanently damaged and may likely provide a short circuitor open circuit.

[0194] To erase the data in the cell the diode is reverse biased: thatis, the anode is brought negative relative to the cathode. Whensufficient potential is applied, the diode breaks down and (e.g.,avalanches, Zeners, or punches through) and strips the charge from theregion 2934. It may be necessary to float the substrate 2930 duringerasing to prevent forward biasing the junction between layer 2932 andthe substrate 2930. Other isolation methods such as shallow-trenchisolation (STI) or silicon-on-insulator (SOI) may be used as well.

B. Three Terminal Cell in the Substrate

[0195] In FIG. 31 the cell incorporates a field-effect transistor havinga source and drain region and a gate 2946. Regions 2941 and 2942 areformed in alignment with gate 2946 in the substrate 2940 as iswell-known in the art. A stack comprising an oxide region 2943, trappingregion 2944 and oxide region 2945 are formed on region 2941. The regions2943, 2944 and 2945 may be the same as regions 2933, 2934 or 2935 ofFIG. 30.

[0196] In this embodiment, rather than forward biasing a diode, apositive potential is applied to gate 2946 and contact 2948 ismaintained positive relative to contact 2947. This is done forprogramming and reading of the cell. To erase the cell, contact 2948 isnegative relative to contact 2947, causing trapped charge to be removedfrom the region 2944. For both the embodiments of FIGS. 30 and 31 it maybe more desirable in some memory arrays to erase an entire array at onetime through the substrate by reverse biasing, say, the region 2941 andsubstrate 2940. If desired, the cells of FIGS. 30 and 31 may be formedabove the substrate rather than in the substrate and/or stacked in threedimensions.

C. Three-Dimensional Embodiment Employing Rail-Stacks

[0197] In U.S. application Ser. No. 09/560,626, filed Apr. 28, 2000, andits co-pending continuation-in-part, U.S. application Ser. No.09/814,727, filed on Mar. 21, 2001 both assigned to the assignee of thepresent invention and entitled “Three-Dimensional Memory Array Method ofFabrication,” a three-dimensional memory array fabricated on thesubstrate and employing rail-stacks is disclosed. The technologydescribed in this patent application may be used to fabricatethree-dimensional charge trapping or storage memories in accordance withthe present embodiment of present invention, as discussed below.

[0198] In FIG. 32, three full levels of a memory array are shown,specifically levels 2950, 2951 and 2952. Each level comprises aplurality of parallel, spaced-apart rail-stacks. Rail-stacks 3 and 5 ofFIG. 32 extend in a first direction and rail-stacks 4 and 6 extend in asecond direction, typically perpendicular to the first direction. Eachof the rail-stacks of FIG. 32 includes a conductor or input/output atthe center of the rail stack and semiconductor regions disposed on bothsides of the conductor. For the embodiment of FIG. 32, first alternaterail-stacks, for instance rail-stacks 3 and 5, are fabricated from ntype polysilicon disposed on the conductors. The second alternaterail-stacks 4 and 6 have p− type polysilicon on the conductors.

[0199] More specifically, referring to rail stack 5, it includes thecenter conductor or input/output 2953, for instance, an aluminum orsilicide conductor, n+ regions 2954 and 2956 disposed on both sides ofthe conductor and n− regions 2955 and 2957 disposed on the regions 2954and 2956, respectively. The n+ regions may be doped to a level of >10¹⁹cm⁻³ and the n− regions to a level of 5×10¹⁶-10¹⁸ cm⁻³. Rail-stacks 4and 6 again include a conductor or input/output, such as conductor 2960with p+ regions disposed on both sides of the conductor shown as p+regions 2961 and 2962 for one of the rail-stacks. The fabrication ofthese regions and the entire set of rail-stacks is described in theabove-referenced application, which is hereby incorporated by referenceherein.

[0200] In the above-referenced application, a blanket layer of ananti-fuse material is used between the rail-stacks. With the presentinvention three blanket layers are used between each level ofrail-stacks. Specifically, layers 2963 are disposed between therail-stacks 5 and 6 and layers 2964 between the rail-stacks 4 and 5. Thelayers 2963 and 2964 correspond to the layers 2933, 2934 and 2935 of,for example, FIG. 30. Thus, layer 2964 comprises a dielectric (e.g.,oxide) layer 2966 which may have a thickness of 1-5 nm, and preferably2-3 nm, a trapping layer 2967 such as a silicon nitride layer which mayhave a thickness of 2-20 nm, and preferably 3-10 nm, and a dielectric(e.g., oxide) layer 2968 which may have a thickness similar to that oflayer 2966. The materials described above for forming the regions 2933,2934 and 2935 of FIG. 30 apply to the layers 2966, 2967 and 2968 of FIG.32.

[0201] A cell in the array of FIG. 32 occurs at the intersection of therail-stacks. For the embodiment of FIG. 32, the storage stack isdisposed between the p and n regions of a diode. That is, the storagestack is embedded in the steering element. For example, conductor 2960provides access to one of the cells through the p region 2961. Thelayers 2963 are disposed between the p region 2961 and n− region 2955.The other contact for this two terminal cell is through region 2954 ontoconductor 2953.

[0202] The cells of FIG. 32 are programmed, read and erased in the samemanner as described above for the cell of FIG. 30.

[0203] With the configuration of FIG. 32 the diodes in adjacent pairs ofmemory array levels “point” to a common conductor. More specifically,referring to FIG. 32, the illustrated cells at memory array level 2950have their cathodes connected to conductor 2953. The illustrated cellsin memory level 2951 also have their cathodes connected to conductors2953. This simplifies fabrication, programming, reading and erasingsince the conductor 2953 serves two sets of cells.

[0204] In the above-referenced application there are several embodimentshaving different rail-stack configurations that may be used to fabricatea three-dimensional array using a preferred storage stack of the presentinvention.

D. Three-Dimensional Embodiment Employing Pillar Diode Structures

[0205] In U.S. Pat. No. 6,034,882 a three-dimensional memory array isdisclosed employing a plurality of levels, each level having parallel,spaced-apart conductors. The conductors at the alternate levels areperpendicular to one another. Pillar structures are formed at theintersection of a conductor in adjacent levels. The structures, asdescribed in the patent, are formed in alignment with the conductors.The fabrication technology described in this patent may be used tofabricate memory arrays employing the cell having a charge storage ortrapping region of the present embodiment.

[0206] Referring to FIG. 33 a single level of the three-dimensionalmemory is illustrated having a conductor or input/output 2981 at onelevel and a conductor 2980 at the next level in the array. A pillarstructure is formed in alignment with the conductors 2980 and 2981. Thispillar structure forms a cell in accordance with the present invention.Specifically, referring to FIG. 33, the cell includes a steering elementcomprising a junction diode comprising the p+ region 2982, n− region2983 and the storage stack. As shown in FIG. 33 the storage stackcomprises a tunnel oxide region 2984, a trapping region 2986 and ablocking oxide 2985.

[0207] As described in the above patent, the conductors 2980 and 2981are shared with cells disposed above and below the single cell shown inFIG. 33.

[0208]FIG. 34 shows another embodiment where again there arespaced-apart, parallel conductors or input/output at one level such asconductor 2991 and parallel, spaced-apart conductors at the next levelsuch as conductor 2990. A pillar structure is again fabricated betweenthe conductors 2990 and 2991 as taught by the above-referenced patent.The difference, however, between the structure of FIGS. 33 and 34 isthat the storage stack comprising the blocking oxide 2993, trappingregion 2994 and tunnel oxide 2995 is disposed between the p and nregions of the diode. Specifically, the p+ region 2992 of the diode isin contact with the blocking oxide 2993 and the n− region 2996 is incontact with the tunnel oxide 2995.

[0209] The thicknesses of the various regions shown in FIGS. 33 and 34and the doping for the polysilicon diode may be similar to embodimentspreviously discussed in this application. The programming, reading anderasing of the structures of FIGS. 33 and 34 are also performed asdescribed above for the other embodiments. For the embodiments of FIGS.32, 33 and 34 the array of cells is disposed above a substrate with theperipheral circuits being formed in the substrate.

[0210] II. Self-Aligned EEPROM TFT Array

[0211] Another cell configuration that differs from pillar configurationis the self aligned TFT. The present inventors have realized that memoryand logic cell area is enlarged by misalignment tolerances that are putinto place to guarantee complete overlap between features on differentlayers. Thus, the present inventors have developed a fully alignedmemory or logic cell structure which does not require misalignmenttolerances. Therefore, such a cell structure has a smaller area per bit(i.e., per cell) and uses fewer mask steps. The fully aligned cellstructure increases array density and decreases die size and cost.Furthermore, by optionally stacking the cells vertically in theZ-direction, the array density is further increased, which leads tofurther decreases in the die size and cost.

[0212] As described with respect to the preferred embodiments of thepresent invention, there are several different ways of achieving a fullyaligned or self-aligned memory or logic cell. In cases of memory orlogic cells containing an EEPROM, full alignment may be achieved by selfalignment of the word line to the control gate. Preferably, the wordline extends substantially parallel to the source-channel-draindirection of the EEPROM, while the bit line extends substantiallyperpendicular to the source-channel-drain direction of the EEPROM. Inthis configuration, bit line contact pads (i.e., source and drainelectrodes) and bit line contact vias are not required because the bitlines may be formed in self alignment with the EEPROM gate(s) directlyon the source and/or drain regions of the EEPROMs. Furthermore, sincethe EEPROMs are fully self aligned, the bit and word lines may have asubstantially planar upper surface, which improves the reliability ofthe device.

[0213] Preferably, the EEPROMs are TFTs arranged in a three dimensionalvirtual ground array (VGA) non volatile flash memory, where eachvertically separated level is separated from an adjacent level by aninterlayer insulating layer. However, the EEPROMs may be formed in asingle level array or in a bulk semiconductor substrate. The preferredaspects of the present embodiment may also be applied to non volatileflash memory architectures other than VGA, e.g., to NOR-type memory andDual String NOR (DuSNOR) memory. Furthermore, the present invention isnot limited to TFT EEPROM flash memory arrays, and also encompassesother semiconductor devices within its scope. For example, the selfaligned transistors may be MOSFETs in a bulk substrate or non-EEPROMTFTs formed over an insulating substrate. These self aligned transistorsmay be used as non-flash EEPROMs (i.e., EEPROMs where each transistor iserased separately), UV erasable PROMs (EPROMs), mask ROMs, dynamicrandom access memories (DRAMs), liquid crystal displays (LCDs), fieldprogrammable gate arrays (FPGA) and microprocessors.

[0214]FIGS. 37-44 illustrate a method of making a TFT EEPROM nonvolatileflash memory array 4001 according to the first preferred embodiment ofthe present invention.

[0215] First, a substrate having an insulating surface (i.e., aSilicon-On-Insulator (SOI) substrate) is provided for the formation ofthe memory array. The substrate may comprise a semiconductor (i.e.,silicon, GaAs, etc.) wafer covered with an insulating layer, such as asilicon oxide or nitride layer, a glass substrate, a plastic substrate,or a ceramic substrate. In a preferred aspect of the first embodiment,the substrate is a monocrystalline bulk silicon substrate that hasreceived prior processing steps, such as forming CMOS (complementarymetal oxide semiconductor) transistors in the substrate. The CMOStransistors may comprise peripheral or driver circuitry for the memoryarray. In the most preferred aspect, the circuitry comprises row andcolumn address decoders, column input/outputs (I/O's), and other logiccircuitry. However, if desired, the driver circuitry may be formed on aninsulating substrate, such as a silicon-on-insulator substrate, a glasssubstrate, a plastic substrate, or a ceramic substrate. Thesilicon-on-insulator substrate may be formed by any conventional method,such as wafer bonding, Separation by Implantation of Oxygen (SIMOX), andformation of an insulating layer on a silicon substrate. After theperipheral circuitry is completed, an interlayer insulating layer 4003is conformally deposited over the circuitry as shown in FIG. 37. Theinterlayer insulating layer 4003 may comprise one or more of anysuitable insulating layers, such as silicon oxide, silicon nitride,silicon oxynitride, PSG, BPSG, BSG, spin-on glass and/or a polymerdielectric layer (such as polyimide, etc.). The interlayer insulatinglayer 4003 is preferably planarized using chemical-mechanical polishing(CMP), but in other embodiments can be planarized by etch back and/orany other means.

[0216] A semiconductor active area layer 4005 is then deposited over theinsulating layer 4003 to complete the SOI substrate. The semiconductorlayer will be used for the transistor active areas. Layer 4005 may haveany desired thickness, such as 20 to 120 nm, preferably 70 nm, and ischosen so that in depletion regime the space charge region below thetransistor gate extends over the entire layer. Preferably, thesemiconductor layer 4005 comprises an amorphous or polycrystallinesilicon layer doped with first conductivity type dopants. For example,layer 4005 may be p− type doped by in-situ doping during deposition, orafter deposition by ion implantation or diffusion.

[0217] If desired, the crystallinity of the semiconductor layer 4005 maybe improved by heating the layer 4005. In other words, an amorphoussilicon layer may be recrystallized to form polysilicon or a grain sizeof a polysilicon layer may be increased. The heating may comprisethermal or laser annealing the layer 4005. If desired, catalyst inducedcrystallization may be used to improve the crystallinity of layer 4005.In this process, a catalyst element such as Ni, Ge, Mo, Co, Pt, Pd, asilicide thereof, or other transition metal elements, is placed incontact with the semiconductor layer 4005. Then, the layer 4005 isthermally and/or laser annealed. During the annealing, the catalystelement either propagates through the silicon layer leaving a trail oflarge grains, or serves as a seed where silicon crystallization begins.In the latter case, the amorphous silicon layer then crystallizeslaterally from this seed by means of solid phase crystallization (SPC).

[0218] It should be noted that the deposition of amorphous orpolysilicon layer 4005 may be omitted if a single crystal SOI substrateis used. In this case, using the SIMOX method, oxygen ions are implanteddeep into a single crystal silicon substrate, forming a buried siliconoxide layer therein. A single crystal silicon layer remains above theburied silicon oxide layer.

[0219] Next, the surface of the active area layer 4005 is preferablycleaned from impurities and a native oxide is removed. A charge storageregion 4007 is then formed on the layer 4005. In the first preferredembodiment of the present invention, the charge storage region 4007comprises an oxide-nitride-oxide (ONO) dielectric triple layer. Thisdielectric comprises a first (bottom) SiO₂ layer, also called a tunneloxide, a charge storage Si₃N_(4-x)O_(1.5x) layer, where x is 0 to 1, anda second (top) SiO₂ layer, also called a blocking oxide. The tunneloxide is either grown by thermal oxidation on the active area layer4005, or deposited over the active area layer by atmospheric pressure,low pressure or plasma enhanced chemical vapor deposition (APCVD, LPCVDor PECVD) or other means. The tunnel oxide has a thickness of 1.5 nm to7 nm, preferably 4.5 nm. The charge storage silicon nitride or siliconoxynitride (Si₃N_(4-x)O_(1.5x)) layer is deposited over the tunneloxide, and its thickness is at least 5 nm, preferably 5-15 nm, mostpreferably 6 nm. The blocking oxide layer is arranged on the surface ofthe charge storage layer and has a thickness of 3.5 nm to 9.5 nm,preferably 5.0 nm. The charge storage and blocking layers may bedeposited by APCVD, LPCVD, PECVD, or other means, such as sputtering.

[0220] It should be noted that different materials and different layerthicknesses may be used as desired. For example, the charge storagelayer need not necessarily be formed from Si₃N_(4-x)O_(1.5x). Forexample, in an alternative aspect of the first embodiment, the chargestorage layer may be formed from a plurality of electrically isolatednanocrystals, such as silicon, tungsten or aluminum nanocrystalsdispersed in a silicon oxide, nitride or oxynitride insulating layer. Ifa nanocrystal charge storage layer is used, then the tunnel and/or theblocking oxide layers may be omitted if desired.

[0221] After the charge storage region 4007 (i.e., the ONO dielectric)formation, a first gate layer 4009 is deposited over the charge storageregion. The first gate layer 4009 may comprise any conductive layer,such as n⁺-doped polysilicon. Such a polysilicon layer may have anyappropriate thickness, such as 50 to 200 nm, preferably 100 nm, and anyappropriate dopant concentration, such as 10¹⁹-10²¹ cm⁻³, preferably10²⁰ cm⁻³.

[0222] If desired, an optional protective layer 4011, such as aprotective silicon oxide layer, is formed on the surface of the firstgate layer 4009. Layer 4011 may have any appropriate thickness, such as,for example 3-10 nm, preferably 5 nm. Materials other than silicon oxidemay be used for layer 4011, if desired.

[0223] A sacrificial blocking layer 4013 is then deposited over theprotective layer 4011. In a preferred aspect of the first embodiment,the blocking layer is made of any conductive or insulating materialwhich may be selectively etched with respect to other layers of thedevice. Preferably, the blocking layer 4013 comprises a silicon nitridelayer. The blocking layer may have any thickness. Preferably theblocking layer 4013 has the thickness that is desired for the wholecontrol gate or an upper part of a control gate, as will be described inmore detail below. For example, layer 4013 has a thickness of 100 to 250nm, preferably 160 nm. FIG. 37 shows the device cross section at thisstage of processing.

[0224] Next, a bit line pattern is transferred to the in process devicewafer or substrate using a reverse bit line mask, as shown in FIG. 38.In this mask, clear areas define the bit lines, and the opaque (i.e.,dark) areas define the space between the bit lines. For example, apositive photoresist layer (not shown in FIG. 38) is formed over theblocking layer 4013 and then exposed through the reverse bit line maskand developed. Of course, if a negative photoresist is used, then theclear and the opaque areas of the mask are reversed.

[0225] The mask features are etched into the blocking nitride 4013, theprotective oxide 4011, and the first gate layer 4009, using thephotoresist layer as a mask, to form a plurality of gate stacks 4015.The ONO dielectric 4007 serves as an etch stop layer. Then, thephotoresist layer is stripped from the patterned gate stacks 4015. Thephotoresist may be removed after the blocking nitride 4013 is etched, inwhich case the nitride may be used as a hard mask for etching the firstgate layer 4009. The gate stacks 4015 include a patterned first gateelectrode 9, an optional protective oxide 4011 and a patterned blockinglayer 4013. If desired, a thin layer of silicon nitride, oxynitiride oroxide is grown to seal the first gate electrode 4009 sidewalls.

[0226] Transistor source and drain regions 4017 are formed byself-aligned ion implantation, using the gate stacks 4015 as a mask. Thephotoresist layer may be left on the gate stacks during thisimplantation or removed prior to the implantation. The ion implantationis carried out through the ONO dielectric 4007. However, if desired, theportions of the ONO dielectric 4007 between the gates 4009 may beremoved prior to the ion implantation.

[0227] Channel regions 4019 of the active layer 4005 are located belowthe gate electrodes 4009. The regions 4017 are doped with a secondconductivity type dopant different from the first conductivity typedopant of the channels 4019. Thus, if the channels 4019 are p− typedoped, then the source and drain regions 4017 are n− type doped, andvice-versa. FIG. 38 shows the device at this stage in the processing.

[0228] It should be noted that in a memory array, the designations“source” and “drain” are arbitrary. Thus, the regions 4017 may beconsidered to be “sources” or “drains” depending on which bit line avoltage is provided. Furthermore, since no field oxide regions arepreferably used in this memory array, each region 4017 is locatedbetween two gate electrodes 4009. Therefore, a particular region 4017may be considered to be a “source” with respect to one gate 4009, and a“drain” with respect to the other gate 4009.

[0229] Next, gate stack sidewall spacers 4021 are formed on thesidewalls of the gate stacks 4015, as shown in FIG. 39. Preferably, thespacers 4021 comprise silicon oxide, if the blocking layer 4013comprises silicon nitride. However, the spacers may comprise anymaterial which allows the blocking layer 4013 material to be selectivelyetched without substantially etching the spacers 4021. For example, thespacers 4021 may comprise silicon nitride if the blocking layer 4013comprises silicon oxide. The spacers 4021 are preferably formed byconformal deposition of a silicon oxide layer over the stacks 4015,followed by an anisotropic oxide etch. The spacer etch process concludeswith an etch process for the ONO dielectric to expose the source anddrain regions 4017. Doping in the source and drain regions 4017 may beincreased at this time by additional self-aligned ion implantation,using the gate stacks 4015 and spacers 4021 as a mask, if desired. Ifso, the implantation before spacer formation may be used to form lightlydoped source/drain (LDD) extensions.

[0230] The salicide process is then used to form suicide regions 4023 inthe silicon source and drain regions 4017 in a self-aligned fashion. Thesalicide process comprises three steps. First a layer of metal, such asTi, W, Mo, Ta, etc., or a transition metal such as Co, Ni, Pt or Pd isblanket deposited over the exposed regions 4017, the sidewall spacers4021 and the blocking layer 4013 of the gate stacks 4015. The device isannealed to perform a silicidation by direct metallurgical reaction,where the metal layer reacts with the silicon in regions 4017 to formthe silicide regions 4023 over regions 4017. The unnreacted metalremaining on the spacers 4021 and the blocking layer 4013 is removed bya selective etch, e.g., by a piranha solution. The silicide regions 4023and the doped silicon regions 4017 together comprise the bit lines 4025.FIG. 39 shows the device at this stage in fabrication.

[0231] A conformal insulating layer 4027 is then deposited to fill thetrenches above the bit lines 4025 and between the sidewall spacers 4021.The insulating layer 4027 may comprise any insulating material, such assilicon oxide, silicon oxynitride, PSG, BPSG, BSG, spin-on glass, apolymer dielectric layer (such as polyimide, etc.), and/or any otherdesired insulating material that is different than the material of theblocking layer 4013. The insulating layer 4027 is then planarized usingchemical-mechanical polishing (CMP), etch back and/or any other means toexpose the upper surface of the silicon nitride blocking layer 4013 onthe gate stacks 4015. FIG. 40 shows the device after the planarizationstep.

[0232] Next, the blocking silicon nitride layer 4013 is etchedselectively without substantially etching the spacers 4021 and theinsulating layer 4027. The protective oxide layer 4011, if present, isthen removed by etching it from the upper surface of the first gateelectrodes 4009 in the stacks 4015. These etching steps form a gatecontact via 4029 above each gate 4009, as shown in FIG. 41. The width ofthe gate contact via 4029 is substantially the same as the width of thefirst gate electrode 4009 because the via sidewalls are the innersidewalls of the sidewall spacers 4021. Therefore, the gate contact vias4029 are self aligned to the gates 4009 because the vias 4029 arebounded by the sidewall spacers 4021 which extend above the gates 4009.No photolithographic masking steps are needed to form the gate contactvias 4029.

[0233] A second gate electrode conductive material 4031 is thendeposited over the entire device, as shown in FIG. 42. Preferably, thematerial 4031 comprises a multilayer stack comprising a first n⁺-dopedpolysilicon layer 4033, a silicide layer 4035 (such as a TiSi or WSi,etc) and a second n⁺-doped polysilicon layer 4037. The polysiliconlayers 4033 and 4037 are preferably 100-300 nm thick, such as 200 nmthick. The suicide layer 4035 is preferably 50 to 100 nm thick, such as60 nm thick. Alternatively, the second gate material can also be formedfrom a single layer of silicide, metal, or any other combination ofheavily doped amorphous or polycrystalline silicon, silicide, and metalthat makes a good ohmic contact with the first gate electrodes 4009.

[0234] Next, a photoresist layer (not shown) is applied over thematerial 4031 and is exposed through the word line mask and developed.The photoresist layer is used as a mask to etch the second gateelectrode material 4031 to form a plurality of word lines 4041. The ONOstack 4007 and the exposed active area layer 4005 are then etched usingthe word lines 4041 as a mask. The photoresist layer may be left on theword lines 4041 during this etching step or it may be removed prior tothis etching step. The bottom insulating layer 4003 under the activearea layer 4005 and the intergate insulating layer 4027 over the bitlines 4025 serve as etch stop layers. Thus, the second gate electrodematerial 4031 is patterned into a plurality of word lines 4041 whichoverlie the intergate insulating layer 4027 as shown in FIG. 43, andinto upper portions 4043 of the first gate electrodes, where thematerial 4031 extends into the vias 4029, as shown in FIG. 44. FIG. 43is a cross section along line A-A in FIG. 42 and FIG. 44 is a crosssection along line B-B in FIG. 42. Therefore, the word lines 4041 areself aligned to the control gates 4009/4043, since a photolithographystep is not required to align the word lines to the gates.

[0235] If desired, the exposed active area 4005 and gate electrode4009/4043 sidewalls may be optionally sealed by growing a thin layer ofsilicon nitride or oxide on them, for example by thermal nitridation oroxidation. This completes construction of the memory array. Aninsulating layer is then deposited, and if necessary planarized, overthe word lines 4041.

[0236] The word line photolithography step does not require misalignmenttolerances, since the word lines are patterned using the same mask asthe charge storage regions 4007 and the active layer 4005 (i.e., channelregions 4019) of each TFT in the cell. Therefore, the word lines 4041are not only self aligned to the control gate 4009/4043 of the TFTEEPROM by being deposited in the self aligned vias 4029, but the wordlines 4041 are also self aligned to the charge storage regions 4007 andthe channel regions 4019 of each memory cell. By using a fully selfaligned memory cell, the number of expensive and time consumingphotolithography steps is reduced. Furthermore, since no misalignmenttolerances for each cell are required, the cell density is increased.Another advantage of the device of the first embodiment is that since athick intergate insulating layer 4027 is located between the bit lines4025 and the word lines 4041, the parasitic capacitance and a chance ofa short circuit between the bit lines and the word lines are decreased.

[0237]FIGS. 45 and 46 illustrate a method of making a TFT EEPROMnonvolatile flash memory array according to the second preferredembodiment of the present invention. The method of the second preferredembodiment is the same as that of the first embodiment illustrated inFIGS. 37-44, except that the sacrificial blocking layer 4013 is omitted.

[0238]FIG. 45 illustrates an in-process semiconductor device 4100according to the second preferred embodiment. The device 4100illustrated in FIG. 45 is at the same stage in processing as the device4001 in FIG. 40. The device 4100 contains the interlayer insulatinglayer 4103, the active layer 4105, the charge storage region 4107 (e.g.,an ONO stack or isolated nanocrystals), source and drain regions 4117,channel regions 4119, silicide regions 4123 and bit lines 4125.

[0239] The gate electrode 4109 of the device 4100 is made thicker thanthe gate electrode 4009 in the first embodiment. For example, the gateelectrode 4109 may have any appropriate thickness, such as 160 to 360nm, preferably 260 nm. Since the blocking 4013 layer is omitted, thegate sidewall spacers 4121 are formed on the patterned gate electrode4109 covered by a protective silicon oxide layer (not shown) after theformation of the source and drain regions 4117. The sidewall spacers4121 extend to the top of the gate electrode 4109. The silicide regions4123 are then formed on the source and drain regions 4117 by depositinga metal layer and reacting the metal layer with the source and drainregions 4117. No silicide is formed on the gate electrode 4109, which iscovered by the silicon oxide protective layer, and on the sidewallspacers 4121. The insulating layer 4127 is then deposited between thesidewall spacers 4121 and over the gate electrodes 4109. Preferably, thelayer 4127 is silicon oxide, but may comprise any other insulatingmaterial, as in the first embodiment. Layer 4127 is then planarized toexpose the upper surface of the gate electrode 4109. The insulatinglayer 4127 is preferably planarized by CMP, but may be planarized byetch back and/or any other means. During the planarization, theprotective silicon oxide layer is also removed to expose the uppersurface of the gate electrode 4109, as shown in FIG. 45.

[0240] Since the selective nitride blocking layer 4013 etch step is notperformed in the second embodiment, the spacers 4121 may be composed ofsilicon nitride, rather than silicon oxide. Silicon nitride spacers areadvantageous because they conform to the underlying topography betterthan oxide spacers. The spacers 4121 and the gate 4109 may act as apolish or etch stop during the planarization of layer 4127.

[0241] After the gate electrodes 4109 are exposed, the memory array ofthe second preferred embodiment is completed just like the array in thefirst preferred embodiment. As in the first embodiment, one or moreconductive layers is/are deposited directly over the tops of thesidewall spacers 4121 and exposed gate electrodes 4109. For example, theconductive layers may comprise a silicide 4135 layer between polysiliconlayers 4133 and 4137. As shown in FIG. 46, the conductive layer(s)is/are then patterned to form a plurality of word lines 4141, whichcontact the exposed gate electrodes 4109. During the same patterningstep, the charge storage region 4107 and the active layer 4105 are alsopatterned, as in the first embodiment. Therefore, the word lines 4141are self aligned to the control gate electrodes 4109, since aphotolithography step is not required to align the word lines to thegates.

[0242] If desired, the exposed active area 4105 and gate electrode 4109sidewalls may be optionally sealed by growing a thin layer of siliconnitride or oxide on them, for example by thermal nitridation oroxidation. This completes construction of the memory array. Aninsulating layer is then deposited, and if necessary planarized, overthe word lines 4141.

[0243] The word line photolithography step does not require misalignmenttolerances, since the word line is patterned using the same mask as thecharge storage regions 4107 and the active layer 4105 of each TFT in thecell. Therefore, the word lines 4141 are not only self aligned to thecontrol gate 4109 of the TFT EEPROM by being deposited directly over theexposed upper surfaces of the gates 4109 and spacers 4121, but the wordlines 4141 are also self aligned to the charge storage regions 4107 andthe channel regions 4119 of each memory cell. By using a fully selfaligned memory cell, the number of expensive and time consumingphotolithography steps is reduced. Since no misalignment tolerances arerequired, the cell density is increased. Furthermore, eliminatingblocking nitride deposition and selective etch steps of the firstembodiment, reduces the step count by three, which simplifies theprocess flow.

[0244]FIG. 47 illustrates a TFT EEPROM nonvolatile flash memory array4200 according to the third preferred embodiment of the presentinvention. The device and method of the third preferred embodiment arethe same as that of the first or the second embodiments illustrated inFIGS. 37-46, except that the charge storage region comprises anelectrically isolated floating gate rather than the ONO stack orisolated nanocrystals as in the first or the second preferredembodiment.

[0245] As shown in FIG. 47, the non-volatile transistor (i.e., the TFTEEPROM) is constructed as a floating-gate field effect transistor. Inthis case, the dielectric triple layer consisting of the ONO stack orthe oxide layer containing electrically isolated nanocrystals isreplaced with a tunnel dielectric, such as tunnel silicon oxide layer4206. The tunnel oxide 4206 has a thickness of 5 to 10 nm, preferably 7nm. The tunnel oxide layer 4206 is formed over the active area 4205, asin the first and second embodiments.

[0246] The first gate electrode 4209 is formed and patterned on thetunnel oxide layer 4206, as in the first and second embodiments.However, in the third embodiment, the first gate electrode 4209comprises a floating gate rather than a control gate. The floating gate4209 is self-aligned to the transistor channel 4219, as in the first andsecond embodiments.

[0247] The device illustrated in FIG. 47 is at the same stage inprocessing as the device in FIG. 42. The device contains the substrate4203, the source and drain regions 4217, channel regions 4219, sidewallspacers 4221 adjacent to floating gate 4209 sidewalls, silicide regions4223, bit lines 4225 and insulating layer 4227.

[0248] The other deviation from the first and second embodiments is theformation of a control gate dielectric 4212 over the floating gate 4209,as shown in FIG. 47. The control gate dielectric may have anyappropriate thickness, such as 8 to 20 nm, preferably 12 nm. The controlgate dielectric 4212 may be grown on the control gate by thermaloxidation or deposited by CVD or other means. The control gatedielectric may comprise silicon oxide, silicon nitride, siliconoxynitride, or an ONO stack. The control gate 4243 and word lines 4241are then deposited and patterned over the control gate dielectric 4212as in the first and second preferred embodiments to complete the deviceshown in FIG. 47. The control gate dielectric 4212 and the control gate4243 are located inside the sidewall spacers 4221.

[0249] FIGS. 48A-C and 49A-C illustrate two alternative preferredmethods of making one TFT (i.e., one cell) in the device 4200 shown inFIG. 47. According to the first preferred method, a gate stack 4215comprising a floating gate 4209, a protective layer 4211 and an optionalsacrificial blocking layer 4213 are formed over the tunnel dielectric4206. The source and drain regions 4217 are implanted into the activearea 4205 using the gate stack 4215 as a mask, such that a channelregion 4219 is formed below the tunnel dielectric 4206. Then, sidewallspacers 4221 are formed over the gate stack 4215. An insulating layer4227 is formed adjacent to the spacers and planarized to expose theblocking layer 4213, as shown in FIG. 48A.

[0250] Then, as shown in FIG. 48B, the protective layer 4211 and theblocking layer 4213 are removed by etching. This forms the gate contactvia 4229. The via 4229 sidewalls are the sidewall spacers 4221 whichextend above the floating gate 4209.

[0251] A control gate dielectric 4212 is then formed, for example, bythermal oxidation, on the exposed floating gate 4209 inside the via 4229as shown in FIG. 48C. Then, one or more conductive layers are depositedover the gate contact via 4229 and the insulating layer 4227. Theselayer(s) are patterned to form a control gate 4243 in the via 4229 and aword line 4241 above layer 4227. The control gate dielectric 4212separates the control gate 4243 from the floating gate 4209.

[0252] According to the second preferred method, a gate stack 4215comprising a floating gate 4209, the control gate dielectric 4212 and asacrificial blocking layer 4213 are formed over the tunnel dielectric4206. The source and drain regions 4217 are implanted into the activearea 4205 using the gate stack 4215 as a mask, such that a channelregion 4219 is formed below the tunnel dielectric 4206. Then, sidewallspacers 4221 are formed over the gate stack 4215. An insulating layer4227 is formed adjacent to the spacers and planarized to expose theblocking layer 4213, as shown in FIG. 49A.

[0253] Then, as shown in FIG. 49B, the blocking layer 4213 is removed byetching to expose the control gate dielectric 4212. This forms the gatecontact via 4229. The via 4229 sidewalls are the sidewall spacers 4221which extend above the floating gate 4209 and the dielectric 4212. Theblocking layer 4213 may consist of a heavily doped polysilicon, in whichcase it may be left in the via 4229, if desired.

[0254] As shown in FIG. 49C, one or more conductive layers are depositedover the gate contact via 4229 and the insulating layer 4227. Theselayer(s) are patterned to form a control gate 4243 in the vias 4229 anda word line 4241 above layer 4227. The control gate dielectric 4212separates the control gate 4243 from the floating gate 4209.

[0255] In the methods of FIGS. 48A-C and 49A-C, the word line 4241 isself aligned to the control gate 4243, to the control gate dielectric4212 and to the floating gate 4209.

[0256]FIG. 50 illustrates a TFT EEPROM nonvolatile flash memory array4300 according to a first preferred aspect of the fourth preferredembodiment of the present invention. The device and method of the fourthpreferred embodiment is the same as that of the third preferredembodiment illustrated in FIG. 47, except that the control gatedielectric is located above the sidewall spacers. Furthermore, theblocking layer 4213 is omitted. As shown in FIG. 50, the sidewallspacers 4221 extend to the top of the floating gate 4209, similar to thedevice of the second preferred embodiment. The control gate dielectric4212 is deposited over the floating gates 4209, the sidewall spacers4221, and the insulating layer 4227. The word line 4241 is thendeposited and patterned over the control gate dielectric 4212, as in thefirst and second preferred embodiments. In the device of FIG. 50, theword line 4241 acts both as a word line and as a control gate. Thus, aseparate control gate may be omitted. The word line 4241 is self alignedto the floating gates 4209. The word line 4241 may comprise one or morelayers, such as the silicide layer 4235 between polysilicon layers 4233and 4237.

[0257]FIG. 51 illustrates a TFT EEPROM nonvolatile flash memory array4300 according to the second preferred aspect of the fourth preferredembodiment of the present invention. The device and method of thispreferred aspect are the same as those illustrated in FIG. 50, exceptthat an upper portion of the floating gate extends above the sidewallspacers. The device illustrated in FIG. 51 is at the same stage inprocessing as the device in FIGS. 47 and 50. As shown in FIG. 51, thedevice contains the interlayer insulating layer 4303, the tunneldielectric 4306, the source and drain regions 4317, channel regions4319, silicide regions 4323, bit lines 4325 and insulating layer 4327.

[0258] The device illustrated in FIG. 51 includes the processing stepsillustrated in FIGS. 48A-B and described above. Thus, a lower portion ofthe floating gate 4309 is exposed in a gate contact via 4329 between thesidewall spacers 4321 which extend above the lower portion of thefloating gate, similar to that shown in FIG. 48B. However, instead offorming a control gate dielectric 4312 in the via 4329, an upper portionof the floating gate 4310 is deposited in the via. The upper portion ofthe floating gate 4310 is formed by depositing a conductive layer, suchas a doped polysilicon layer, over the vias 4329, the spacers 4321 andthe insulating layer 4327, such that it contacts the exposed lowerportion of the floating gate 4309 in the via 4329. The conductive layeris patterned using photolithography into an upper floating gate portion4310 such that it extends vertically above the sidewall spacers 4321.Preferably, the conductive layer also extends horizontally above thespacers 4321. Thus, the upper gate portions 4310 have a “T” shape. Then,the control gate dielectric 4312 is formed on the exposed upper surfaceof the upper portion of the floating gate 4310 by thermal growth, CVDand/or various other deposition techniques (such as sputtering, etc.).One or more conductive layers 4333, 4335, 4337 are then deposited overthe control gate dielectric 4312 and are patterned into word lines 4341.The conductive layers may be, for example, a silicide layer 4335sandwiched between doped polysilicon layers 4333, 4337, as in the firstpreferred embodiment. In the fourth preferred embodiment, the word lines4341 serve as the control gates of the TFTs. Since the top surface ofthe floating gate 4309/4310 in the fourth embodiment is larger than inthe third embodiment, the area between the floating gate and the controlgate/word line is increased in the TFT of the fourth embodiment comparedto the third embodiment. The increase in area between the floating gateand the control gate/word line is advantageous because it increases thecapacitive coupling between the floating gate and the control gate/wordline.

[0259] In a preferred aspect of the fourth embodiment, the top surfaceof the upper portion of the floating gate 4310 is textured or roughenedto further increase the capacitive coupling between the floating gateand the control gate/word line. For example, at least the upper portionof the floating gate 4310 may be made of hemispherical grain silicon(HSG), or the upper surface of the floating gate may be roughened byetching or coarse polishing. In other words, the upper portion of thefloating gate may be textured or roughened similar to the texturing orroughening methods used to texture or roughen bottom conductive platesof DRAM capacitors.

[0260] While the first through fourth preferred embodiments describe andillustrate a TFT EEPROM nonvolatile flash memory array, the presentinvention should not be considered to be so limited. For example, ratherthan a self aligned word line in a TFT EEPROM array, any gate line maybe self aligned to a MOSFET (i.e., metal oxide semiconductor fieldeffect transistor) gate according to the preferred embodiments of thepresent invention. Furthermore, the EEPROM array may be formed in a bulksilicon substrate rather than over an interlayer insulating layer.

[0261] The first through the fourth preferred embodiments describe andillustrate a cross-point array of word lines and bit lines at ahorizontal level and a method of making thereof. Each memory cellconsists of a single programmable field effect transistor (i.e., TFT),with its source and drain connected to the j^(th) bit line and the(j+1)^(st) bit line, respectively, and a control gate being eitherconnected to or comprising the k^(th) word line. This memory arrangementis known as the NOR Virtual Ground (NVG) Array (also referred to asVGA). If desired, the memory array may also be arranged in non volatileflash memory architectures other than VGA, such as NOR-type memory orDual String NOR (DuSNOR) memory, for example. The DuSNOR architecture,where two adjacent cell strings share a common source line but usedifferent drain lines, is described in K. S. Kim, et al., IEDM-95,(1995) page 263, incorporated herein by reference. The DuSNOR memory maybe fabricated using the same process as the VGA memory, except that anadditional masking step is used to pattern the active area layer toseparate the drain regions of adjacent cells. The process sequence ofthe first through third preferred embodiments of the present inventionrequires only two photolithographic masking steps. One masking step isfor gate patterning/self aligned bit line formation. The other maskingstep is for word line patterning. The methods of the preferredembodiments of the present invention exploit self-alignment to reducealignment tolerances between the masks. The memory cell area achievedwith the foregoing process is about 4F², where F is the minimum featuresize (i.e. 0.18 microns in a 0.18 micron semiconductor process). Theterm “about” allows for small deviations (10% or less) due tonon-uniform process conditions and other small deviations from desiredprocess parameters. If the charge storage medium used in the transistoris not conductive, e.g., it is formed from nitride or oxy-nitride (i.e.using the ONO charge storage medium), or electrically isolatednanocrystals, the localized nature of charge storage can be exploited tostore two bits per cell. In this case, the effective cell area per bitequals about 2F².

[0262] The NVG array of the first through fourth preferred embodimentsis very suitable for vertical stacking of horizontal planar NVG arrays.FIG. 52 illustrates a three dimensional memory array 4400 according to afifth preferred embodiment of the present invention. The threedimensional memory array contains a three dimensional array of TFTEEPROMs made according to the first, second, third or fourth preferredembodiment. Each TFT EEPROM contains a channel 4419, source and drainregions 4417, a control gate 4443, control gate sidewall spacers (notshown for clarity in FIG. 52) and a charge storage region 4407 betweenthe channel and the control gate 4409. The charge storage region maycomprise an ONO dielectric, isolated nanocrystals or a floating gate.

[0263] The memory array also contains a plurality of bit line columns4425, each bit line contacting the source or the drain regions 4417 of aplurality of TFT EEPROMs. The columns of the bit lines 4425 extendsubstantially perpendicular to the source-channel-drain direction of theTFT EEPROMs (i.e., a small deviation from the perpendicular direction isincluded in the term “substantially perpendicular”). It should be notedthat the columns of the bit lines 4425 may extend substantiallyperpendicular to the source-channel-drain direction of the TFT EEPROMsthroughout the entire array 4400 or only in a portion of the array 4400.The bit lines in each device level are shaped as rails which extendunder the intergate insulating layer. The bit lines include the burieddiffusion regions formed during the source and drain doping steps andthe overlying silicide layers. The source and drain regions are formedin the bit lines where the word lines intersect (i.e., overlie) the bitlines and the doped regions are located adjacent to the EEPROM channelregions.

[0264] The memory array also includes a plurality of word line rows4441. Each word line contacts the control gates 4443 of a plurality TFTEEPROMs 4400 (or the word lines comprise the control gates). The rows ofword lines extend substantially parallel to the source-channel-draindirection of the TFT EEPROMs (i.e., a small deviation from the paralleldirection is included in the term “substantially parallel”). It shouldbe noted that the rows of the word lines 4441 may extend substantiallyparallel to the source-channel-drain direction of the TFT EEPROMsthroughout the entire array 4400 or only in a portion of the array 4400.The plurality of word lines 4441 are self aligned to the control gates4443 of the array of TFT EEPROMs (or the word lines themselves comprisethe control gates). If floating gates, but not control gates areincluded in the array, then the word lines are self aligned to thefloating gates and to the control gate dielectric.

[0265] Each device level 4445 of the array is separated and decoupled inthe vertical direction by an interlayer insulating layer 4403. Theinterlayer insulating layer 4403 also isolates adjacent word lines 4441and adjacent portions of the active areas 4405 below the respective wordlines 4441 in each device level 4445. The effective cell area per bit inthe resulting three dimensional memory array is about 2F²/N, where N isthe number of device levels (i.e., N=1 for a two dimensional array andN>1 for a three dimensional array). The array of nonvolatile memorydevices 4400 comprises a monolithic three dimensional array of memorydevices. The term “monolithic” means that layers of each level of thearray were directly deposited on the layers of each underlying level ofthe array. In contrast, two dimensional arrays may be formed separatelyand then packaged together to form a non-monolithic memory device.

[0266] Each cell in one level 4445 of the memory array can be formedusing only two photolithographic masking steps. However, additionalmasking steps may be needed to form contacts to the bit lines 4425. In asixth preferred embodiment of the present invention, a conductive layeris formed over the array of memory devices. The conductive layer is thenpatterned to form a plurality of word lines or word line contact layersand at least one bit line contact layer which contacts at least one ofthe plurality of the bit lines. Thus, a separate bit line contactdeposition and patterning step may be avoided, since the same conductivelayer may be patterned to form the word lines/word line contacts and thebit line contacts. Of course, if desired, the word lines/word linecontacts and the bit line contacts may be made from different materialsand/or patterned using different masks.

[0267]FIG. 53 illustrates a bit line contact 4447 according to onepreferred aspect of the sixth preferred embodiment. In FIG. 53, a firstdoped polysilicon layer 4433 is formed over the inter-gate insulatinglayer 4427. A bit line contact via 4449 is then formed in the insulatinglayer 4427 in which a top portion of the bit line 4425 is exposed. Asilicide layer 4435 and a doped polysilicon layer 4437 are thendeposited, such that the silicide layer 4435 contacts the bit line 4425through the via hole. The layers 4433, 4435 and 4437 are thenphotolithographically patterned using the same mask to form both theplurality of word lines 4441 and a plurality of bit line contacts 4447.An upper interlayer insulating layer 4403 is then formed over the wordlines 4441 and bit line contacts 4447. Word line contact vias 4451 andbit line contact layer contact vias 4453 are formed in the insulatinglayer 4403 for formation of further contacts. It should be noted thatthe word lines 4441 and the bit line contact layer 4447 are not limitedto the materials described. The layers 4441 and 4447 may comprise one ormore polysilicon, silicide or metal layers. Furthermore, while the gateline 4441 and the contact 4447 are located in the same level of thedevice, the contact 4447 may extend into a lower level of the array tocontact a bit line or a word line in the lower level of the array, ifdesired.

[0268]FIG. 54 illustrates a bit line contact 4547 according to anotherpreferred aspect of the sixth preferred embodiment. In this embodiment,at least one bit line contact via 4549 extends through at least oneinterlayer insulating layer 4503 between different levels of the array.In FIG. 54, the word line 4541 is first patterned and an interlayerinsulating layer 4503 is deposited thereon. Word line contact vias 4551and bit line contact vias 4549 are formed in the insulating layer 4503.The bit line contact via 4549 extends through the intergate insulatinglayer 4527 to the bit line 4525, which comprises the doped region 4417and the silicide region 4423.

[0269] Then one or more conductive layers, such as silicide layer 4555and doped polysilicon layer 4557 are deposited on the interlayerinsulating layer 4503 and in the vias 4551 and 4549. The one or moreconductive layer(s) 4555, 4557 are then photolithographically patternedusing the same mask to form both a word line contact 4559, the bit linecontact 4547, and plurality of word lines in the memory layer above thememory layer shown.

[0270] The word line and bit line contacts can reach down to lowerlevels, e.g., every other lower level, or several lower levels at thesame time. Thus, in FIG. 54, the bit line contact 4547 and the word linecontact 4559 are formed in the N+1 level of the array, and extend to theword lines 4541 and the bit lines 4525 in the Nth level of the array.The word line contacts and bit line contacts connect the word lines andthe bit lines with the peripheral circuits located in the semiconductorsubstrate below the first device level of the array (or locatedelsewhere in the array, such as above or within the array, butpreferably at least in part vertically integrated or aligned with thearray). Landing pads are made in level N+1 conductor for the next levelcontacts.

[0271]FIGS. 55 through 61 illustrate a method of making a TFT EEPROMnonvolatile flash memory array according to the seventh preferredembodiment of the present invention. The method of the seventh preferredembodiment starts in the same way as that of the first, second, third,or fourth embodiments illustrated in FIGS. 37-51, except that asacrificial dummy block which holds the place of the gate electrode isused in the process. A transistor formed by this method is called areplacement-gate transistor. The array made by the seventh preferredembodiment may be formed as three dimensional array shown in FIG. 52,having an effective cell area per bit of about 2F²/N.

[0272] As in the previously described embodiments, the process startswith a deposition of a semiconductor active area, such as an amorphoussilicon or polycrystalline silicon layer 4605 over an interlevelinsulating layer 4603, as shown in FIG. 55. Then, a plurality ofsacrificial dummy blocks 4604 are formed over the active layer 4605, asshown in FIG. 56. The sacrificial dummy blocks 4604 may comprise one ormore materials, at least one of which may be selectively etched withrespect to the material of an intergate insulating layer 4627 to beformed later. For example, if the intergate insulating layer 4627comprises silicon oxide, then the dummy blocks may comprise siliconnitride, silicon oxynitride, polysilicon or other materials which may beselectively etched with respect to silicon oxide.

[0273] Preferably, the active layer 4605 comprises amorphous silicon andthe dummy blocks 4604 are formed of a material which is deposited at atemperature below 600° C. to avoid recrystallizing the amorphous siliconlayer 4605 into a polysilicon layer with a small grain size. Forexample, the dummy blocks 4604 may be formed by depositing a lowtemperature PECVD silicon nitride layer over the active layer 4605 andpatterning the silicon nitride layer into a plurality of dummy blocks4604 using photolithography.

[0274] In a preferred aspect of the seventh embodiment, the dummy blocks4604 comprise a plurality of layers, including a sacrificial channeldielectric layer 4667, a sacrificial gate layer 4669, and a protectiveoxide layer 4671, as shown in FIG. 55. Layers 4669 and 4671 arepatterned using a reverse bit line mask, similar to that illustrated inFIG. 38 of the first preferred embodiment, to form the dummy blocks4604, as shown in FIG. 56. Since all layers 4667, 4669, 4671 above theactive layer are sacrificial, lower quality materials may be used forthese layers. For example, low temperature silicon oxide (LTO) or PECVDsilicon oxide may be used for the channel dielectric layer 4667. Thus,layer 4667 may be deposited at a low temperature (i.e., below 600° C.)to avoid recrystallizing the amorphous silicon active layer 4605 into apolysilicon layer with a small grain size. If desired, all layers of thedummy blocks 4604 may be deposited at temperatures below 600° C. In thiscase, the amorphous state of layer 4605 is preserved until a subsequentsalicide formation on the source and drain regions 4617. The silicide4623 on the source and drain regions 4617 may act as a catalyst forlateral crystallization of amorphous silicon in the source and drainregions 4617 to form a polycrystalline silicon active layer 4605 with alarge grain size.

[0275] Subsequently, TFT source and drain regions 4617 are implantedinto the active layer 4605 using the dummy blocks as a mask. The channellayers 4619 are located in layer 4605 between regions 4617 and below theblocks 4604. If the dummy blocks 4604 contain a polysilicon layer, thenpreferably, sidewall spacers 4621 are formed on the dummy block 4604sidewalls to separate silicide from the source/drain junctions, toprevent subsequent silicide formation on the dummy blocks and toincrease flexibility in source/drain engineering. The spacers 4621 maybe composed of silicon oxide or silicon nitride, or two differentlayers, as shown in FIG. 57. If desired, an additional implantation maybe performed into the source and drain regions 4617 using the blocks4604 and spacers 4621 as a mask. If the dummy blocks 4604 do not containpolysilicon (i.e., are composed of silicon nitride), then the spacers4621 may be omitted.

[0276] A metal layer, such as Ti, W, Mo, Ta, etc., or a transition metalsuch as Co, Ni, Pt or Pd is blanket deposited over the exposed regions4617 and the dummy blocks 4604. The device is annealed to perform asilicidation by direct metallurgical reaction, where the metal layerreacts with the silicon in regions 4617 to form the silicide regions4623 over regions 4617, as shown in FIG. 58. The unnreacted metalremaining on the dummy blocks 4604 is removed by a selective etch, e.g.,by a piranha solution. The active layer 4605 is then recrystallized bylaser or thermal annealing using the silicide regions 4623 as acatalyst. Alternatively, if desired, the active layer 4605 may berecrystallized simultaneously with the silicide 4623 formation, or theactive layer 4605 may be recrystallized by laser or thermal annealingbefore the formation of the dummy blocks 4604.

[0277] After the formation of the buried bit lines 4625 which containthe source and drain regions 4617 and the silicide 4623 regions, aconformal intergate insulating layer 4627 is deposited between and abovethe dummy blocks 4604. Preferably, layer 4627 comprises silicon oxide(HDP oxide), as in the other preferred embodiments. The layer 4627 isthen planarized by CMP and/or etchback to expose the top portions of thedummy blocks 4604. For example, if the dummy blocks 4604 contain asilicon oxide protective layer 4671 and silicon oxide spacers 4621, thenthese layers may be removed together with the top portion of layer 4627during planarization. In this case, the top portions of the sacrificialgates 4669 are exposed after planarization, as shown in FIG. 58.

[0278] Next, the dummy blocks 4604 are selectively etched (i.e.,removed) without substantially etching the intergate insulating layer4627. For example, if the dummy blocks 4604 include the sacrificialpolysilicon gates 4609, then these sacrificial gates 4609 areselectively etched without substantially etching the spacers 4621 andthe intergate insulating layer 4627. If the dummy blocks include asacrificial gate dielectric layer 4667, then this layer 4667 can beremoved using plasma etch back or wet etch methods. As shown in FIG. 59,a plurality of vias 4629 are formed in locations where the dummy blocks4604 were previously located.

[0279] After the surface of the active layer 4605 above the channelregions 4619 is exposed by removing the dummy block materials, the“real” or permanent gate dielectric material is immediately grown and/ordeposited on the exposed regions. Preferably, this dielectric comprisesa charge storage region 4607 selected from the ONO triple layer or theplurality of electrically isolated nanocrystals, as shown in FIG. 60.Alternatively, this dielectric may comprise a tunnel dielectric 4606 ifthe TFT EEPROM contains a floating gate 4609, as shown in FIG. 61. Thecharge storage layer 4607 is located on the bottom of the vias 4629above the channel regions 4619. The charge storage layer 4607 alsocontains vertical portions located on the sidewalls of the intergateinsulating layer 4627 (or on the sidewalls of the spacers 4621, if thespacers are present) and horizontal portions located above the intergateinsulating layer 4627, as shown in FIG. 60.

[0280] Subsequently, a conductive material is deposited over theintergate insulating layer 4627 and the charge storage regions 4607. Theconductive material may comprise polysilicon or a combination ofpolysilicon 4633, 4637 and silicide 4635 layers, as in the otherembodiments. The conductive material fills the vias 4629 and overliesthe charge storage layer 4607. The conductive material is then patternedto form a plurality of word lines 4641, as in the other embodiments. Theactive layer 4605 and the charge storage layer 4607 is then patternedusing the word lines 4641 as a mask as in the other embodiments. Theportions of the word lines 4641 located in the vias 4629 comprise thecontrol gates 4609 of the TFT EEPROMs, as shown in FIG. 60. If afloating gate TFT EEPROM is desired, then a floating gate 4609 and acontrol gate dielectric 4612 may be formed in the vias 4629 prior toforming the control gates/word lines 4641, as shown in FIG. 61.

[0281] In an eighth preferred embodiment of the present invention, theTFTs in a plurality of the levels of the three dimensional array of FIG.52 undergo a recrystallization and/or a dopant activation step at thesame time. This reduces the device fabrication time and cost.Furthermore, if each level of the array were subjected to a separatecrystallization and/or dopant activation annealing, then the lowerlevels would undergo more annealing steps than the upper levels. Thismay lead to device non uniformity because the grain size may be largerin the active areas of the lower levels and/or the source and drainregions may have a different dopant distribution in the lower levelsthan in the upper levels.

[0282] Thus, in a first preferred aspect of the eighth embodiment,amorphous silicon or polysilicon active areas of TFTs in a plurality oflevels are recrystallized at the same time. Preferably, TFTs in alllevels are recrystallized at the same time. The recrystallization may beeffected by thermal annealing in a furnace or by rapid thermal annealing(RTA) in an RTA system. The thermal annealing may be carried out at 550to 800° C. for 6-10 hours, preferably at 650 to 725° C. for 7-8 hours.

[0283] Furthermore, since a silicide layer 4423 contacts the source anddrain regions 4417, the silicide may act as a catalyst forrecrystallization, especially if nickel, cobalt or molybdenum silicideis used. The metal atoms diffuse though the active areas of the TFTs,leaving behind large grains of polysilicon. Thus, recrystallizing theamorphous silicon or polysilicon active areas after depositing the bitline metallization leads to larger grains and allows the use of lowerrecrystallization temperatures, such as 550 to 650° C. Furthermore, noseparate metal deposition and patterning for metal inducedcrystallization is required. Thus, each level of the array may besubjected to a recrystallization anneal after the bit line metallizationis formed for this level. Alternatively, all levels of the array may besubjected to a recrystallization anneal after the bit linemetallizations for every level of the array have been formed.Furthermore, in an alternative aspect of the eighth embodiment, silicideformation step and the recrystallization steps may be carried out duringthe same annealing step for each level of the array.

[0284] In a second preferred aspect of the eighth embodiment, the dopedregions in a plurality of levels are activated at the same time.Preferably, the doped regions in all of the levels are activated at thesame time. The doped regions comprise the TFT source and drain regionsas well as any other doped region formed in the three dimensional array.Preferably, the doped regions are activated by subjecting the array toan RTA treatment. However, if desired, the activation may be carried outby thermal annealing at about 700 to about 850° C. for 20 to 60 minutes.The activation may be carried out before or after the crystallizationanneal.

[0285] In a third preferred aspect of the eighth embodiment, therecrystallization and dopant activation are carried out in the sameannealing step of a plurality of levels or for all the levels of thearray. The annealing step should be conducted at a sufficiently hightemperature and for a sufficient length of time to activate the dopantsand to recrystallize the TFT active areas, without causing the sourceand drain region dopants to diffuse into the channel regions of theTFTs. Preferably, the combined recrystallization and dopant activationannealing step comprises an RTA treatment.

[0286] In a fourth preferred aspect of the eighth embodiment, an extraphotolithographic masking step is provided to form crystallizationwindows used to deposit the crystallization catalyst material. Forexample, as shown in FIG. 62, the material 4722 used to form sidewallspacers 4721 is patterned using a separate photolithographic mask toform the crystallization windows 4701. Thus, in the replacement-gatetransistor method shown in FIGS. 55-61, the crystallization windows 4701are formed in the low temperature oxide (LTO) layer used to makesidewall spacers after the reverse bit line pattern is etched into theprotective oxide 4771 and the sacrificial gates 4769. Crystallizationmask features are etched into the oxide layer 4722 to clear the surfaceof the active layer 4705. Simultaneously, sidewall spacers 4721 areformed on the sacrificial gates 4769. Then, the photoresist (not shown)is stripped. FIGS. 63 and 64 illustrate cross-sections along lines A-Aand B-B in FIG. 62, respectively. If desired, the crystallizationwindows may also be added to the process of the first through the fourthembodiments. Such windows would be formed during the formation of thesidewall spacers in those embodiments.

[0287] Next, a catalyst, such as Ni, Ge, Fe, Mo, Co, Pt, Pd, Rh, Ru, Os,Ir, Cu, Au, a silicide thereof, or other transition metal elements ortheir silicides, is deposited. The catalyst comes in contact with theamorphous silicon active layer 4705 only in the open windows 4701. Thecatalyst material may be deposited as a solid layer or as a catalystsolution. Alternatively, the catalyst may be ion implanted or diffusedinto the active layer 4705. Then, the device is annealed for severalhours at a temperature below 600° C., preferably at 550° C. This lowanneal temperature is preferred to minimize spontaneous nucleation inthe amorphous silicon. Polysilicon grains in the present embodimentstart growing from the seed regions in the windows 4701 and growlaterally. At the completion of anneal, the grain boundaries 4702 arealigned as shown in FIG. 65. Then, the catalyst is removed. A solidcatalyst layer may be removed by selective etching, while catalyst atomsin the recrystallized polysilicon may be removed by gettering, such asby annealing the device in a chlorine containing gas. The LTO oxidelayer 4722, which comprises the boundaries of crystallization windows4701, is then removed by selective etching, and the device is completedas in the other embodiments. It should be noted that the word lines (WLin FIGS. 62 and 65) are subsequently formed over the regions where thecrystallization windows 4701 used to be formed. Since thecrystallization begins in the windows 4701, the grain boundaries 4702which are parallel to the word lines are located away from the windowregions, in the regions of the active layer 4705 between the word lines.These regions of the active layer 4705 between the word lines areremoved after the formation of the word lines. Therefore, since thechannel regions of the TFTs are located below the word lines, these TFTchannel regions contain fewer grain boundaries, and substantially nograin boundaries which are parallel to the word lines.

[0288] III. Rail Stack TFTs

[0289] The following preferred embodiments provide an array of TFTs witha charge storage region, such as EEPROM TFTs, arranged in a rail stackconfiguration. The embodiments described herein are in the context of anon-volatile reprogrammable semiconductor memory and methods offabrication and utilization thereof. Those of ordinary skill in the artwill realize that the following detailed description of the embodimentsof the present invention is illustrative only and is not intended to bein any way limiting. Other embodiments of the present invention willreadily suggest themselves to such skilled persons having the benefit ofthis disclosure. Reference will now be made in detail to implementationsof the present invention as illustrated in the accompanying drawings.The same reference indicators will be used throughout the drawings andthe following detailed description to refer to the same or like parts.

[0290] In the interest of clarity, not all of the routine features ofthe implementations described herein are shown and described. It will,of course, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

[0291] The present embodiment is directed to a two- or, more preferably,a three-dimensional many-times-programmable (MTP) non-volatile memory.The memory provides a bit cell size of 2F²/N where F is the minimumfeature size (e.g., 0.18 microns in a 0.18 micron semiconductor processand 0.25 microns in a 0.25 micron semiconductor process) and N is thenumber of layers of devices in the third (i.e., vertical) dimension.Thus, for a 0.18 micron process with 8 devices stacked vertically, theeffective bit cell size projected on the substrate is only about 0.0081square microns. As a result, a 50 mm² chip with 50% array efficiency ina 0.18 micron technology and with 8 layers of memory devices would haveapproximately 3.1 billion memory cells for a capacity of approximately386 megabytes with two bits stored per cell and 193 megabytes with onebit stored per cell. The three-dimensional versions of the memory use anextension to three dimensions of the “virtual ground array” commonlyused with single crystalline silicon memory devices. The preferredmemory process architecture uses N+ doped polysilicon railsperpendicular to rail stacks of P− doped polysilicon/charge trappinglayer/N+ polysilicon in a cross-point array forming NMOS transistormemory devices with a SONOS charge trapping layer which may beduplicated vertically. Of course a PMOS memory can also be made.

[0292] Adjacent pairs of N+ polysilicon rails and a rail stack of P−doped polysilicon/charge trapping layer/N+ doped polysilicon define thesource, drain and gate, respectively, of a unique NMOS memory device.Programming and erasing change the threshold voltage of this NMOS. Withhot electron injection programming, two bits per NMOS can be stored anderasing can be performed either with hot hole injection or withFowler-Nordheim tunneling.

[0293] Turning now to FIG. 80, a method of integrating memory devices inaccordance with a specific embodiment of the present invention into amulti-level array of storage cells will now be described. Thefabrication starts by providing a substrate 5180 on which the multilevelarray of storage devices is to be formed. Substrate 5180 will typicallyinclude a lightly doped monocrystalline silicon substrate 5182 in whichtransistors such as metal oxide semiconductor (MOS) transistors areformed. These transistors can be used as, for example, accesstransistors or they can be coupled together into circuits to form, forexample, charge pumps or sense amps for the fabricated memory devices.Substrate 5180 will typically also include multiple levels ofinterconnects and interlayer dielectrics 5184 used to couple transistorsin substrate 5182 together into functional circuits. The top surface5186 of substrate 5180 will typically include an insulating layer orpassivation layer to protect the underlying transistors andinterconnects from contamination. The top surface 5186 will typicallycontain electrical contact pads to which multilevel arrays of memorydevices of the present invention can be electrically coupled in order tomake electrical contact with the transistors in silicon substrate 5182.In an embodiment of the present invention, the memory devices arephysically isolated and separated from the single crystalline substrateby multiple levels of interconnects and dielectric 5184. The top surfaceof passivation or insulating layer 5186 will typically be planarized toenable uniform and reliable fabrication of multiple levels of the memorydevices of the present invention. According to the present invention,the memory devices are physically separated from monocrystalline siliconsubstrate 5182. In an alternative embodiment of the present invention,memory devices can be fabricated on a glass substrate 5180 such as usedin flat panel displays.

[0294] A process of forming a multilevel array of thin film transistor(TFT) memory devices above the substrate in accordance with anembodiment of the present invention begins by blanket depositing a firstconductor layer 5188 over surface 5186 of substrate 5180. Conductor 5188can be any suitable conductor such as, but not limited to, titaniumsilicide, doped polysilicon, or a metal such as aluminum or tungsten andtheir alloys formed by any suitable technique. Conductor layer 5188 isto be used as, for example, a bitline or a wordline to couple a row orcolumn of memory devices together. Next, a planarization is performed bydepositing or growing an insulating layer such as a silicon oxide overconductor layer 5188 to fill spaces between bit lines. A conventionalchemical mechanical polishing (CMP) step completes the planarization andexposes the bitlines.

[0295] Turning now to FIG. 66, a specific embodiment of the presentinvention is illustrated in front perspective view. In this embodiment,a 2-dimensional memory array 5040 includes a first plurality ofspaced-apart conductors such as N+ doped polysilicon bit lines 5042,5044, 5046, 5048 disposed in a first direction a first height over (notin contact with) the substrate (not shown). A second plurality ofspaced-apart “rail stacks” 5050, 5052 are disposed in a second directiondifferent from the first direction (and preferably orthogonally) at asecond height above the substrate so that they are above bit lines 5042,5044, 5046 and 5048 and in contact therewith at intersection points5054, 5056, 5058, 5060, 5062, 5064, 5066, 5068. Each rail stack 5050,5052 in this embodiment includes at least a layer of P− dopedpolysilicon 5070 which may be formed, for example, by depositing anamorphous silicon film by chemical vapor depositing (CVD) and which isin situ doped with P type impurities (e.g., Boron) to a dopant densityof about 1×10¹⁶ to about 1×10¹⁸ atoms/cm³. The amorphous silicon filmscan then be converted into polycrystalline silicon through a subsequentanneal step. Alternatively, instead of in situ doping, undoped siliconcan be grown or deposited and then implanted or diffused with dopants.Over layer 5070 is disposed a charge trapping layer 5072 comprising acharge trapping medium as discussed below, and a conductive wordline5074 which may comprise N+ doped (or P+ doped) polysilicon disposed overthe charge trapping layer 5072. A planarized oxide material (not shownin FIG. 66) may be deposited in the spaces between and above adjacentbit lines and rail stacks. A conventional chemical mechanical polishing(CMP) process may be used to accomplish the planarization.

[0296] The memory array structure of FIG. 66 can now be easilyextrapolated to three dimensions. To do this, the CMP planarized oxidelayer over wordlines 5050, 5052 is used. The planarized isolation layer(or interlayer insulating layer) prevents shorting one set of wordlineswith the next set of bit lines. Then another layer of bit lines 5042,5044, 5046, 5048 is constructed over the isolation layer followed by anoxide deposition and a CMP step, followed by a deposition of another setof wordlines. This process can be repeated a number of times, asdesired. In accordance with a specific embodiment of the presentinvention, eight layers of memory array (or more) are stacked one uponanother to provide 8 times the bit density of the non-three-dimensionalversion.

[0297] Turning now to FIG. 67, another specific embodiment of thepresent invention is illustrated. In this embodiment a 2-dimensionalarray 5076 includes an isolation layer 5078 electrically separating itfrom the substrate (not shown). The isolation layer may be anyconventional isolation/insulation layer such as a silicon oxide. Overisolation layer 5078 is disposed a plurality of spaced-apart bit lines5080, 5082, 5084, 5086. Bit lines 5080, 5082, 5084, 5086 are preferablyformed of N+ doped polysilicon although P+ doped polysilicon could alsobe used as could any suitable electrical conductor. A deposition step isused to fill the regions 5088, 5090, 5092 between adjacent bit lines5080, 5082, 5084, 5086 with a filler material. The filler material mustbe an electrical insulator. Again, silicon oxide is convenient althoughother materials could also be used. A CMP step is then used to planarizeand expose the bit lines. A layer 5094 of a semiconductor material suchas P− doped polysilicon is then disposed over and in contact with bitlines 5080, 5082, 5084, 5086. An ONO layer 5096 is disposed over thesemiconductor layer 5094 and a conductive wordline 5098 is disposed overONO layer 5096. In accordance with a presently preferred embodiment, thebit lines 5080, 5082, 5084, 5086 and the wordlines 5098 are formed of N+doped polysilicon. When thermally processed, N+ out diffusion regions5100, 5102, 5104, 5106 are formed in P− doped semiconductor layer 5094.The channels 5108, 5110, 5112 between adjacent N+ out diffusion regionsbecome channels of NMOS transistors whose threshold voltages arecontrolled by the presence or absence of trapped charge in the nitridelayer of ONO dielectric stack 5096.

[0298] Those of ordinary skill in the art will realize thatsemiconductors of the opposite conductivity types may also be used.Where a conductor other than doped polysilicon is used for the wordlinesand bit lines it will be necessary to form a doped region insemiconductor layer 5094 in some way other than by out diffusion.

[0299]FIG. 68 is a top plan view of the memory array of FIG. 67. Asshown in FIG. 68, the wordlines 5098 are arranged over the bit lines5080 in a cross point array. While the wordlines and the bitlines arearranged perpendicular (i.e., at a 90 degree angle) to each other inFIG. 68, an angle between the wordlines and bitlines may differ from 90degrees. Furthermore, outside the boundaries of the memory array, thewordlines and the bitlines may change directions and even be parallel toeach other. Furthermore, the term “rail stack” or “rail” preferablyrefers to conductors arranged in straight lines. However, if desired,the rails or rail stacks may have bends, twists or turns, if desired.

[0300] Turning now to FIG. 69 the memory array of FIG. 67 isextrapolated to a monolithic three-dimensional array. The term“monolithic” means that layers of each level of the array were directlydeposited on the layers of each underlying level of the array. Incontrast, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device. Each devicelevel 5076 is preferably identical to that shown in FIG. 67 and anisolation layer (i.e., interlayer insulating layer) 5078 separates eachlevel. A single cell (i.e., a TFT EEPROM) 5099 is delineated by thedashed line in FIG. 69. The cell 5099 is located in device level “j” atthe intersection of word line (n,j) and bit lines (m,j) and (m+1,j).

[0301] Turning now to FIG. 70, another specific embodiment of thepresent invention is illustrated. In this embodiment, an array of bottomgate TFTs is formed. A two-dimensional memory array 5114 is disposedabove a substrate. An isolation layer 5116 is disposed to separatememory array 5114 from the substrate (not shown) or another level ofmemory array (not shown). A plurality of spaced-apart wordlines 5118 aredisposed over isolation layer 5116. Over wordline 5118 are disposed afilm of a charge trapping medium 5120, such as an ONO dielectric stack.Over the charge trapping medium 5120 is disposed a plurality ofspaced-apart bitlines 5122, 5124, 5126, 5128. In the space 5130, 5132,5134 between bit lines 5122, 5124, 5126, 5128 is disposed a film ofsemiconductor material 5136. This may be deposited into spaces 5130,5132, 5134 or it may be deposited or grown over charge trapping medium5120 and then masked and etched so that bitlines 5122, 5124, 5126, 5128are formed after it has been formed. This version of the memory arrayapproximates turning the design of FIG. 69 upside down. In this way, thebitlines are trenches that would be filled by N+ doped polysilicon.Prior to filling, n− type implantation is carried out to form the MOSdevices' sources and drains. In addition, a refractory metal may be usedat the bottom of the trenches instead of dopant to form the sources anddrains.

[0302] Turning now to FIG. 71 the memory array of FIG. 70 isextrapolated to a monolithic three-dimensional array. Each level 5114 ispreferably identical to that shown in FIG. 70 and an isolation layer5116 separates each level.

[0303] Turning now to FIG. 72, another specific embodiment of thepresent invention is illustrated, where each bit line acts as a bit linefor TFTs in two device levels. In this embodiment a memory array 5140includes a lower word line 5142 and an upper word line 5144. Bitlines5146, 5148, 5150, 5152 are disposed between upper wordline 5144 andlower wordline 5142. In a manner similar to that of FIG. 67 and FIG. 69,an upper semiconductor film 5154 is disposed between bitlines 5146,5148, 5150, 5152 and upper wordline 5144. Lower semiconductor film 5156is disposed between bitlines 5146, 5148, 5150, 5152 and lower wordline5142. Out diffusion regions are formed adjacent to bitlines 5146, 5148,5150, 5152 in upper semiconductor film 5154 and lower semiconductor film5156. A lower charge storage medium film 5158 is disposed between lowerwordline 5142 and lower semiconductor film 5156. An upper charge storagemedium film 5160 is disposed between upper wordline 5144 and uppersemiconductor film 5154. Notice that in this embodiment the layers arecopied in a mirror image fashion.

[0304] Turning now to FIG. 73, the memory array of FIG. 72 isextrapolated to a monolithic three dimensional array. Each device level5140 may be thought of as containing two word lines and two TFT activeregions and a plurality of bit lines disposed between the activeregions. Alternatively, each device level may be thought of as a singlewordline 5142 being disposed between two TFT active regions. Thus, eachdevice level contains either one wordline level and two bitline levelsor one bitline level and two wordline levels. Each TFT active regionshares both a bitline and a wordline with another TFT active regiondisposed in a different horizontal plane.

[0305] An alternative bottom gate TFT embodiment is illustrated in FIGS.81A-81H. The approach of FIGS. 81A-81H is somewhat similar to that ofFIG. 70. Layer 5116 is an isolation layer such as an oxide separatingthe memory array structure 5114 from other memory array levels or fromthe substrate. Layer 5118 is a conductive wordline layer. Layer 5120 isan O—N—O dielectric stack. Layer 5136 is a film of semiconductormaterial (p− type when the wordlines and bitlines are N+ polysilicon).

[0306] In FIG. 81B an oxide layer 5190 is deposited or grown. In FIG.81C the oxide layer 5190 is masked with a mask 5192 (i.e., a photoresistmask). In FIG. 81D the unmasked portions of the oxide layer 5190 areetched in a conventional manner.

[0307] In FIG. 81E the mask 5192 is removed and semiconductor layer 5136is implanted with n− type ions to form an N+implantation region 5194 ateach opening in the oxide layer 5190 as illustrated in FIG. 81F. In FIG.81G an N+ layer 5196 is deposited to fill gaps in the oxide and formbitline 5198 of N+ material in contact with N+ implantation regions 5194so as to provide a contact with the O—N—O layer 5120. In FIG. 81H the N+layer 5196 is CMP planarized as shown to form the bitlines 5198, tocomplete an NMOS TFT array. Of course a PMOS TFT array may beconstructed by reversing the conductivity types of the layers anddopants. A multilayer version of the memory array of FIGS. 81A-81H canbe constructed by forming additional device levels separated by anisolation layer.

[0308] Another alternative embodiment of a top gate TFT array isillustrated in FIGS. 82A-82I. In FIG. 82A an oxide or isolation layer5200 is disposed above a substrate (not shown). In FIG. 82B a layer ofsemiconductor material of a first conductivity type 5202 is disposedover oxide layer 5200. The semiconductor material may be P− dopedamorphous silicon. Over this in FIG. 82C is deposited a hard nitrideCMP-stop layer 5204 to stop the CMP process from polishing into layer5202.

[0309] In FIG. 82D the memory array under construction is masked withmask 5206, as a photoresist mask. In FIG. 82E an etch is being carriedout to form apertures or trenches 5208 as shown in FIG. 82F. In FIG. 82Ga conductive layer 5210 is deposited, such as n+ doped polysilicon. InFIG. 82H this layer 5210 is CMP polished down leaving N+ bitlines 5212with P− doped regions 5214 between them. After thermal processing, outdiffusion regions 5216 are formed as shown in FIG. 82I. Furthermore, theamorphous silicon layer 5202 is recrystallized into a polysilicon layer.

[0310] In FIG. 82I a local charge storage film 5218 is disposed overbitlines 5212 and a conductive film 5220 is disposed over local chargestorage film 5218. The conductive film 5220 is patterned to form awordline. The charge storage film 5218 is also patterned to form railstacks which include the wordline and the charge storage film.

[0311] The charge storage medium film used herein (also referred toherein as a “local charge storage film”) needs to be able to retain alocalized charge, i.e., it must not laterally conduct. In oneembodiment, a charge trapping layer may be formed in a dielectric stack5160 as shown in FIG. 77. For example, the charge storage medium can bea dielectric stack 5160 comprising a first oxide layer 5162 adjacent toa polysilicon film 5164, a nitride layer 5166 adjacent to the firstoxide layer 5162 and a second oxide layer 5168 adjacent to the nitridelayer 5166 and adjacent to a polysilicon control gate 5170. Such adielectric stack 5160 is sometimes referred to as an ONO stack (i.e.,oxide-nitride-oxide) stack. Other suitable charge trapping dielectricfilms such as silicon implanted or silicon-rich oxides can be used ifdesired.

[0312] The charge storage medium film may alternatively be formed from aplurality of electrically isolated nanocrystals 5172 as shown in FIG.78. Nanocrystals are small clusters or crystals of a conductive materialwhich are electrically isolated from one another. An advantage of theuse of nanocrystals for the charge storage medium is that because theydo not form a continuous film, nanocrystals are self isolating.Nanocrystals 5172 enable multiple self-isolating charge storage areas tobe formed.

[0313] Nanocrystals 5172 can be formed from conductive material such assilicon, tungsten or aluminum. In order to be self isolating thenanocrystals must have a material cluster size less than one-half thepitch of the cell so that floating gates from vertically andhorizontally adjacent cells are isolated. That is, the nanocrystals ormaterial clusters 5172 must be small enough so that a single nanocrystal5172 cannot bridge vertically or horizontally adjacent cells. Siliconnanocrystals can be formed by depositing silicon in a manner wherebysilicon has a very high surface diffusivity relative to its stickingcoefficient. For example, silicon nanocrystals can be formed by chemicalvapor deposition (CVD), by decomposing silane (SiH₄) at a very lowpressure, in a range of about 1 millitorr to about 200 millitorr, at atemperature in a range of about 250’ to about 650° C. In such a process,a very thin deposition, in a range of about 50 Å to about 250 Å, willform little islands of silicon. If H₂ is included with silane during thedeposition, higher pressures can be utilized and still obtainnanocrystals. In an alternative embodiment of the present invention,metal nanocrystals such as aluminum nanocrystals, can be formed bysputtering from a metal target at a temperature near the meltingtemperature of the metal, so that the metal agglomerates and formsnanocrystals. Tungsten nanocrystals can be formed by chemical vapordeposition at very low pressures by utilizing a reactant gas mixcomprising a tungsten source gas such as WF₆ and germane (GeH₄). Instill yet another embodiment of the present invention, a continuous filmof floating gate material can be deposited and then caused toprecipitate (by heating) to cause islands to form in the film.

[0314] It is to be appreciated, that although nanocrystals are preferredfor the floating gate, because of their self isolating quality, thefloating gate can be formed from a continuous film such as, but notlimited to, a metal such as tungsten or a silicon film such aspolycrystalline or amorphous silicon doped to the desired conductivitytype (typically N+ silicon). If a continuous film is used as a localcharge storage film, the film would be anisotropically etched at thistime to remove portions of it in order to electrically isolate strips ofthe film.

[0315] Similarly, small pieces of floating gate material, such asheavily doped polysilicon, may form a local charge storage medium whenembedded in an insulator such as an oxide layer.

[0316] An issue with using N+ out diffusion in a multi-level device isthat the various levels will be exposed to different thermal processing.That is, the bottom layer will be exposed to each thermal processingstep while the top layer is only exposed to the last thermal processingsteps. Since it is undesirable to have the MOS memory transistorsexhibiting substantially different performance characteristics dependingupon level in the array and it is undesirable to allow lateral diffusionto swamp the MOS memory transistors, care needs to be given to thethermal budget and mechanisms for forming source/drain regions. Where N+doping is used for the bitline and P− doping for the semiconductor film,it is possible to use antimony as the dopant instead of phosphorous asantimony exhibits a smaller diffusivity than phosphorous. It is alsopossible to engineer the dopant profile in the bitline polysilicon toallow different out diffusions. This is shown in FIG. 76 in schematicrepresentation. After polysilicon dopant diffusion is characterized forvarious thermal budgets for the polysilicon depositions, one can easilydetermine how far away the N+ in situ doped material should be from theP− doped body region as a function of memory level within the array.Antimony could also be used here and could be directly implanted, ifdesired. In FIG. 76, the bitlines denoted (a) are closer to the toplevel of the memory array than are the bitlines denoted (b). In otherwords, bitlines (a) are located above bitlines (b) in the array. Duringthe thermal treatment, the dopants in the bitlines will diffuse upwardsthroughout the entire bit lines and outdiffuse into the P− polysiliconlayer to form the source and drain regions. Thus, the source and drainregions in plural levels will be evenly doped.

[0317] Turning now to FIG. 69, to program the first bit in the selectedcell in FIG. 69, WL(n,j) is pulsed high (9-13V, high impedance) whileBL(m,j) is grounded and BL(m+1,j) is pulsed high (3-8V, lowerimpedance). All BL's to the left of BL(m,j) on the j^(th) level are heldat ground while all BL's to the right of BL(m+1,j) on the j^(th) levelare held at the same voltage as BL(m+1,j). All other WL's on the j^(th)level are held at ground to make sure that all other MOS devices betweenBL(m,j) and BL(m+1,j) are off. All other BL's and WL's on all otherlayers can be left floating. This means that the selected cell MOSdevice is uniquely on and powered to optimize hot carrier generation andprogramming into the charge trapping dielectric close to the drain(defined by BL(m+1,j)).

[0318] To read the first bit, BL(m+1,j) is now the source and BL(m,j) isthe drain. The former is grounded and the latter is raised to a readvoltage (^(˜)50 mV to 3V, preferably 1-3V) while WL(m,j) is pulsed to aread voltage (^(˜)1-5V). Again, all BL's to the left of BL(m,j) are heldat the same potential as BL(m,j) and all BL's to the right of BL(m+1,j)are grounded. All other WL's on the same level are grounded to shut offall other MOS devices between the same two BL's. All other BL's and WL'son all other levels can be left floating.

[0319] To program and read the second bit in the same cell, the voltageson BL(m,j) and BL(M+1,j) are reversed compared to the above.

[0320] Notice that the body region of the MOS memory transistor isfloating and can be made thin (defined by the deposition tool, e.g.,preferably several hundred Angstroms). By making this region thin,snapback of the device can be avoided and so rapid increase inprogramming currents can also be avoided.

[0321] Erasing of the memory can take place in blocks and may employ acombination of slow Fowler-Nordheim tunneling and hot hole injection.The erase current will be small since the MOS body is floating resultingin very little band-to-band tunneling and avalanche breakdown. Erase cantake place with the wordlines either grounded or held negative (⁻−5V)and all bitlines held at some positive voltage. The erase procedure willtake over 100 ms and can be done at each memory level up to the fullmemory at one time.

[0322] Non-selected bits with common wordline should be able towithstand the programming voltage on the wordline for a worst caseperiod of time. FIG. 74 shows this in schematic detail in one level ofthe matrix.

[0323] If each bit (i.e. half cell) needs time t to program and thereare N cells on each WL then, in a worst case, a programmed bit wouldexperience (2 N−1)t of time where the programming voltage would beapplied to the WL. The gate stress program disturb would be fine if anyprogrammed cell did not shift its Vt by a certain “minimal” amount.Since programming is achieved using hot electrons, the times andvoltages are short and small respectively compared to voltages and timesneeded to tunnel out of charge traps. In addition, the total stress onany one bit may be effectively reduced by floating unselected bitlinesduring the programming of the selected cell. In this way, only theselected bitline at ground will experience a true full programmingvoltage across the dielectric(s).

[0324] Non-selected bits with a bitline in common with the selected bitshould be able to withstand the programming voltage on the drain for aworst case period of time. FIG. 75 shows this in schematic detail wherea cross section along a bitline is shown.

[0325] Again, if there are M cells on any one bitline and it takes timet to program any one bit, then the worst case drain stress on aprogrammed bit will be (M−1)t in time. So the Vt shift in a programmedbit after experiencing such a stress should be minimal.

[0326] Read disturb or “soft write” occurs if the hot carriers generatedduring a read of the cell are sufficient to eventually (over 10 yearslifetime) program a previously erased (unwritten) bit. Acceleratedtesting is usually carried out here to make sure that the read voltagesrequired do not shift the threshold voltage of a neutral cell by morethan a minimal amount.

[0327] In the devices set forth above, N+ or P+ doped polysilicon shouldbe doped to a dopant density of about 1×10¹⁹ to 1×10²¹ atoms/cm³ andhave a thickness preferably in a range of about 500 Å to about 1000 Å.P− or N− doped semiconductor films should be doped to a dopant densityof about 1×10¹⁶ to about 1×10¹⁸ atoms/cm³.

[0328] It is to be appreciated that each of the memory devices shown canbe made of opposite polarity by simply reversing the conductivity typeof each of the silicon regions and maintaining dopant concentrationranges. In this way, not only can NMOS devices be fabricated, but alsoPMOS devices can be formed if desired. Additionally, the silicon filmsused to form the device may be recrystallized single crystal silicon orpolycrystalline silicon. Additionally, the silicon film can be a siliconalloy film such as a silicon germanium film doped with n− type or p−type conductivity ions to the desired concentration.

[0329] Where it is desired to increase the lateral conductivity ofpolysilicon wordlines and bitlines, a layer of a conductive metal may bedeposited in the wordline or bitline as illustrated in FIG. 79. In FIG.79 bitline 5174 is formed of polysilicon 5176 which is heavily N+ doped.This makes it electrically conductive. To further reduce electricalresistance, a layer of a refractory electrically conductive metal suchas titanium 5178 may be disposed within the bitline 5174, or on one ormore surface of the polysilicon 5176. When subjected to normal siliconprocessing temperatures the titanium forms a silicide with thepolysilicon that is highly conductive in a lateral direction.

[0330] IV. Flash Memory Array in a Rail Stack Configuration

[0331] In the previous embodiments, the TFTs were arranged in a virtualground array (VGA). In a VGA illustrated in the previous embodiments,the programming of each EEPROM occurs by hot carrier injection. In hotcarrier injection, a voltage is placed across a diode (i.e., between asource and a drain of a TFT EEPROM). The hot carriers (i.e., hotelectrons and holes) that are travelling from source to drain throughthe channel of the TFT EEPROM are injected into the charge storageregion which is disposed adjacent to the channel. This procedure is arelatively high power event.

[0332] For low power portable applications where both program/erase andread power are important, a flash nonvolatile memory usingFowler-Nordheim tunneling (“FN tunneling”) for both program and erasemay be used. FN tunneling results from applying a voltage across adielectric. Thus, in a TFT EEPROM, a voltage is applied between acontrol gate and a source and/or a drain) region of the TFT, for writingand erasing the TFT EEPROM. This is in contrast with hot carrierinjection programming, where a voltage is applied between the source andthe drain regions.

[0333] A flash memory array which uses FN tunneling for program anderase is advantageous because thousands of bits in such a flash memoryarray may be programmed at the same time.

[0334] Also, FN tunneling is a very efficient way of programming sincemost (close to 100%) of the current goes to program the device. This isin contrast with hot carrier injection where only about 1-2% of thesource-drain current goes to program the device.

[0335] Thus, in a preferred embodiment of the present invention, chargestorage devices, such as TFT EEPROMs, are arranged in a flash memoryarray configuration. The TFT EEPROMs may be arranged in the pillar,self-aligned TFT or rail stack configurations of the previousembodiments. Preferably, the TFT EEPROMs are arranged in the rail stackconfiguration.

[0336] The VGA is not compatible with FN tunneling since the wholechannel polysilicon inverts along the length of the pulsed-high wordline and will then program cells in addition to the one that needsprogramming. Therefore, the FN tunneling rail stack (crosspoint) flasharray differs from the VGA in that in the FN tunneling array the activepolysilicon layer is patterned into polysilicon islands to allow FNtunneling programming. Thus, an extra photolithographic masking step isadded to the process of making the rail stack array during which thepolysilicon active layer is etched into islands in each device cell. Thesame photoresist mask can be used to define (i.e., etch) the chargestorage regions in each cell.

[0337]FIG. 83A illustrates a flash memory array in a rail stackconfiguration according to a preferred embodiment of the presentinvention. FIG. 83B shows a cross sectional view along line B-B in FIG.83A.

[0338] In FIG. 83A, the flash memory array 5230 is preferably formedover a planarized interlayer insulating layer 5231, such as a CMPplanarized silicon oxide layer. Layer 5231 is formed over a substrate(not shown) as in the previous embodiments. Each device of the array(shown by dashed lines 5232 in FIG. 83A) is thus a TFT because it isformed over an insulating layer.

[0339] The array 5230 contains a first plurality of spaced-apartconductive bit lines 5233 disposed at a first height above the substratein a first direction. The array also contains a second plurality ofspaced-apart rail-stacks 5235. The rail stacks are disposed at a secondheight in a second direction different from the first direction.Preferably, the bit lines 5233 and the rail stacks 5235 are arrangedperpendicular to each other. The TFT EEPROM 5232 is formed at theintersection of the rail stacks 5235 and the bit lines 5233.

[0340] Each rail-stack 5235 includes a plurality of semiconductorislands 5237, which comprise the active regions of the TFT EEPROMs 5232.One surface of the islands 5237 is in contact with the bit lines 5233.Each rail stack 5235 also includes a conductive word line 5239 and acharge storage region 5241 disposed between a second surface of thesemiconductor islands 5237 and the word line 5239.

[0341] The semiconductor islands 5237 preferably comprise polysilicon ofa first conductivity type (i.e., P− or N−). However, the islands maycomprise amorphous silicon if desired. The polysilicon islands 5237include source and drain regions 5243 of a second conductivity type(i.e., N+ or P+). The source and drain regions 5243 are located atcontacting intersections between the bit line conductors 5233 and therail stacks 5235.

[0342] The bit lines 5233 preferably comprise polysilicon of the secondconductivity type (i.e., N+ or P+). The bit lines 5233 contact thesource and drain regions 5243. Preferably, the source and drain regionsare formed by outdiffusion of dopants from the bit lines. Furthermore,an optional metal or a metal suicide layer (not shown in FIG. 83A) maybe disposed in contact with the bit lines 5233 to increase theconductivity of the bit lines. The space between said spaced-apart bitline conductors 5233 is filled with a planarized insulating fillermaterial 5245, such as silicon oxide.

[0343] The charge storage regions 5241 may comprise a dielectricisolated floating gate, electrically isolated nanocrystals or an O—N—Odielectric stack, as in the previous embodiments. An exemplary arrayhaving a dielectric isolated floating gate is illustrated in FIGS. 83Aand B. Thus, in the example of FIGS. 83A and B, the charge storageregion 5241 comprises a polysilicon floating gate 5247 between a tunneldielectric 5249, such as a silicon oxide layer, and a control gatedielectric 5251 (also known as the intergate or interpoly dielectric)made of a material such as silicon oxide or an ONO layer stack.

[0344] As shown in FIGS. 83A and B, the lateral sides 5253 of the tunneldielectric 5249 and the floating gate 5247 are aligned to the lateralsides 5255 of the semiconductor islands 5237. The control gatedielectric 5251 extends between the semiconductor islands 5237 andcontacts the planarized insulating material 5245 between thesemiconductor islands 5237. If desired, the floating gate 5247 may bemade from hemispherical grain polysilicon which has a textured surfaceto maximize the control gate to floating gate coupling. Alternatively,the coupling may be increased by increasing the floating gate height, byforming horns or protrusions in the floating gate, or by roughening thefloating gate surface.

[0345] The word line 5239 comprises a polysilicon layer of a secondconductivity type (i.e., N+ or P+) and a metal or a metal silicide layerin contact with the polysilicon layer. The word line 5239 acts as acontrol gate of the TFT EEPROM in locations where it overlies the chargestorage regions 5241. Thus, formation of a separate control gate foreach TFT is not required.

[0346] In one preferred aspect of this embodiment, the rail stacks 5235are disposed above the bit lines 5233, as shown in FIGS. 83A and B.However, if desired, the rail stacks 5235 may be disposed below the bitlines 5233 in each device level, as described with respect to FIG. 70 ina previous embodiment (i.e., bottom gate TFT EEPROMs are formed).

[0347] As shown in FIG. 83B, the word line 5239, the charge storageregions 5241 and the semiconductor islands 5237 (i.e., the rail stacks5235) are aligned in a plane 5256 perpendicular to the substrate andparallel to a source to drain direction. The rail stacks 5235 areseparated by a second planarized insulating layer 5257, such as siliconoxide.

[0348] While the flash memory array may comprise a two dimensionalarray, preferably, the flash memory array comprises a monolithic threedimensional array comprising a plurality of device levels. For example,three device levels are shown in FIG. 83A. The device levels areseparated by an interlayer insulating layer 5259, such as a siliconoxide layer. If desired, layers 5257 and 5259 may comprise the samesilicon oxide layer which is deposited above and between the rail stacks5259, and then planarized by CMP.

[0349] To program the selected TFT EEPROM 5232, either its drain bitline or its source bit line 5233 (or both) are grounded while thepositive programming voltage is applied to the selected word line 5239adjacent to the device 5232 (which is a high impedance node). All otherword lines on the same device level are grounded while all other bitlines on the same level device can float or are placed at a slightpositive voltage. This means that only the selected cell 5232experiences the programming voltage across it. Through capacitivecoupling, the floating gate 5247 is pulled high while the source and/ordrain 5243 are grounded. Electrons tunnel to the floating gate 5247 fromthe source and/or drain 5243 and an inversion channel is formed in thesilicon channel 5237. The current to program such a cell to get athreshold voltage shift of about 5V in approximately one millisecond isseveral picoamps.

[0350] To erase the cell, the same bit lines 5233 can be grounded and anegative voltage pulse is applied to the selected word line 5239. Allother word lines can either be grounded or can float. All other bitlines float or are placed at a slight negative voltage. A plurality (orall) of EEPROM cells in the array can be erased at the same time bypulsing a plurality of word lines to a high negative value while all bitlines are grounded. Alternatively, the selected wordline is groundedwhile the selected cell's bit lines are pulsed positive. All other wordlines float or are pulsed slightly positive while all the other bitlinesare grounded.

[0351] Programming and erasing using FN tunneling alone allows use oflow current programming and erasing, which lends itself to “massiveparallelism” in programming and erasing. Therefore, many cells 5232 canbe programmed in parallel. For example, to get 5V shift, one thousandcells would need about 2 nA in total current and would program in about1 microsecond per cell, average. During programming and erasing, theparasitic leakage currents are small because no large voltages areplaced across polysilicon diodes (i.e., source/channel/drain junctions).During reading, the parasitic leakage currents are also small becausesource to drain voltages are also small. A programming voltage of 10-20Vmay be used to program the cells. In the above approach of FIGS. 83A andB, a small cell size is achieved. However, only positive thresholdvoltages (for NMOS TFT EEPROMs shown in FIGS. 83A and B) are attainable,since otherwise large amounts of parasitic bit line to bit line leakageresults. In order to allow both positive and negative threshold voltagesin each cell, an access transistor (i.e., a TFT MOSFET) is added to eachcell in a second preferred aspect of the flash memory array, as shown inFIG. 84.

[0352]FIG. 84 illustrates a built-in access transistor 5261 in each cellwhose threshold voltage can be set to a slight positive value. By usingthe access transistor 5261, the actual cell transistor (i.e., the TFTEEPROM 5232) can have a negative threshold voltage without introducingbit line leakage and avoiding special erase-and-check algorithms thatprevent over-erase. Furthermore, the access transistor can also reducethe defect-based TFT band-to-band tunneling leakage that may occur atnegative gate voltages and could be problematic in programmed cells(i.e., floating gate full of electrons), (see S-H Hur et al., “A Poly-SiThin-Film Transistor EEPROM Cell with Folded Floating Gate”, IEEE Trans.Elect. Dev., vol. 46, pp. 436-438, February 1999, incorporated herein byreference).

[0353] As shown in FIG. 84, the semiconductor islands 5237 containadjacent channel regions 5263, 5265 of the access transistor 5261 andthe EEPROM 5232, respectively, between the common source 5243A and drainregions 5243B. The word lines 5239 form control gates of the EEPROMs andgate electrodes of the access transistors. An insulating layer 5251forms a common control gate dielectric of the EEPROM and a gateinsulating layer of the access transistor. The floating gate 5247 and atunnel dielectric 5249 are located between the word line 5239 and thechannel region 5265 of the EEPROM 5232.

[0354] To program the floating gate 5247 of a cell 5232/5261, its sourcebit line 5233A is grounded, its drain bit line 5233B floats, and a highpositive voltage pulse is applied to the selected cell's word line. Thistunnels electrons to the floating gate. All other bit lines on the samedevice level are left floating or are placed at a slight positivevoltage while all other word lines on the same level are grounded. Toread, the selected cell's word line is pulsed to a read voltage of abovethe access transistor's threshold voltage while the cell's source bitline is grounded and drain bit line is set at a low positive voltage,such as 1 to 3 V. All other bit lines at the same level are leftfloating or grounded while all word lines at the same level aregrounded. To erase the cell, its word line is pulsed to a high negativevalue while its source bit line is grounded. To erase the whole array,all word lines can be pulsed to a high negative value while all sourcebit lines are grounded.

[0355] In another preferred aspect of the flash memory array, a gate todrain offset region 5267 is provided to reduce TFT band-to-band defectrelated drain leakage, as shown in FIG. 85. Thus, in the example of FIG.85, the word line 5239 and the charge storage region 5241 are offsetapart from the drain region 5243B. A thick insulating layer 5269 islocated between the semiconductor islands 5237 and the word lines 5239in the offset region 5267. The floating gates 5247, the tunneldielectric 5249 and the control gate dielectric 5251 have alignedlateral sides 5253A and B. Only one of the lateral sides 5253A isaligned to the lateral side 5255A of the semiconductor islands 5237. Theislands 5237 have a greater width than the floating gates 5247, thetunnel dielectric 5249 and the control gate dielectric 5251.

[0356] If desired, ONO or isolated nanocrystal charge storage regionsmay be used instead of the floating gate charge storage regions in theembodiments of FIGS. 84 and 85. Furthermore, the devices of FIGS. 84 and85 may be formed in a bottom gate configuration (i.e., with the bitlines above the word lines) if desired.

[0357] In the flash memory array of FIGS. 83A and B, each cell size perbit is about 8F²/N to about 10 F²/N, where F is a minimum feature sizeand N is a number of device levels in the array. In the flash memoryarray of FIGS. 84 and 85, each cell size per bit is about 9 F²/N toabout 11F²/N. Thus, a cell size per bit of about 8 F²/N to about 11F²/N,may be achieved. This cell size compares favorably with cell sizes ofcommercially available flash memory arrays, which range from 7.7 F² to13.9 F². If the access transistors and contacts are factored in theeffective cell size of the commercially available devices, then due toredundancy, their cell size ranges from 9.8 F² to 19.2 F². However, whenthe flash memory array of the present embodiment is formed as a threedimensional array (i.e., N>1), then the cell size per bit of the flashmemory array of the present embodiment is significantly smaller thanthat of the prior art. For example, for N=2, the cell size is about 4 F²to about 5.5 F². For N>2, the cell size is even smaller.

[0358] The method of making the flash memory array of FIGS. 83-85 isillustrated in FIG. 86. FIGS. 86A-D illustrate a method of making theflash memory array where the word lines are disposed above the bit linesin each device level. A plurality of spaced-apart bit line conductors5233 are formed at a first height above the substrate (not shown) byetching a first conductive layer using a first photoresist mask. The bitline conductors 5233A and B extend in a first direction, as shown inFIG. 86A. Preferably, the bit lines comprise polysilicon and metal ormetal silicide layers. A first insulating layer 5245 is deposited aboveand between the bit line conductors 5233A, B. The insulating layer 5245is planarized by CMP until the top surface of the bit line conductors5233A, B is exposed.

[0359] A stack of layers including a first semiconductor layer 5237 anda charge storage film are deposited on the exposed bit line conductors5233A, B and the planarized insulating layer 5245, as shown in FIG. 86B.Layer 5237 may be an amorphous silicon or a polysilicon layer. In FIG.86B, the charge storage film comprises a tunnel dielectric layer 5249and a floating gate polysilicon layer 5247. Alternatively, the chargestorage film may be an ONO stack or dielectrically isolatednanocrystals.

[0360] A second photoresist layer (not shown) is formed on the stack andphotolithographically patterned into a mask. Using this photoresistlayer as a mask, the stack of layers 5237, 5249 and 5247 is etched toform a plurality of first rail stacks 5271 (only one such rail stack isshown in FIG. 86C for clarity). The first rail stack 5271 extends in thesame or substantially the same direction as the bit line conductors 5233in a plane parallel to the substrate. Each of the first rail stacks 5271contains a semiconductor rail 5237 and a charge storage region rail5247/5249. The first rail stacks 5271 have at least one aligned lateraledge 5253/5255. In FIG. 86C, the first rail stacks 5271 have two suchaligned lateral edges since each first rail stack is patterned using thesame photoresist mask, which is removed after the etching step.

[0361] If floating gate type EEPROMs are to be formed, then the controlgate insulating layer 5251 is deposited over the first rail stacks 5271and in the spaces 5273 between the first rail stacks, as shown in FIG.86D. Thus, layer 5251 extends beyond the lateral edges of the first railstacks 5271. If an ONO or isolated nanocrystal type EEPROMs are to beformed, then the semiconductor layer 5237 would be deposited andpatterned into first rail stacks 5271 after deposition. Then the ONO orthe nanocrystal containing layer would be deposited over the patternedfirst rail stacks 5271, followed by the deposition of a conductive layer5239 for the wordline.

[0362] A second conductive layer 5239 is deposited over the control gateinsulating layer 5251. Preferably, layer 5239 comprises polysilicon andmetal suicide sublayers. A third photoresist mask (not shown) is formedover the second conductive layer 5239. The second conductive layer 5239,the control gate dielectric 5251 and the first rail stacks 5271 are thenetched to form a plurality of second rail stacks 5235, as shown in FIG.86D. The second rail stacks comprise the patterned second conductivelayer which forms the word line 5239, charge storage region islands5247/5249/5251 and the semiconductor islands 5237.

[0363] The source 5243A and drain 5243B regions are formed byoutdiffusing dopants of a second conductivity type (i.e., N+ or P+) intothe semiconductor islands 5237 of a first conductivity type (i.e., P− orN−) from the first plurality of spaced-apart conductors. The source anddrain regions may be formed at any time during the fabrication sequenceafter the semiconductor layer 5237 is deposited on the bit lineconductors 5233A, 5233B. For example, the device may be annealed afterthe formation of the second rail stacks 5235 to outdiffuse the dopantsinto the source and drain regions and to recrystallize the amorphoussilicon layer 5237 into a polysilicon layer (or to increase the layer5237 grain size). The outdiffusion anneal and the crystallization annealmay occur during the same or during separate heating steps. For example,the recrystallization anneal may take place right after layer 5237 isdeposited.

[0364] The side surfaces of the second rail stacks 5235 are aligned in aplane perpendicular to the substrate and parallel to a direction whichextends from the source 5243A to the drain 5243B of the TFT EEPROM 5232,as shown in FIG. 83B. The control gate dielectric 5251 is disposedbetween the word line 5239 and the first insulating layer 5245. Sincethe control gate dielectric is part of the first rail stacks 5235, thecontrol gate dielectric 5251 is aligned in a plane perpendicular to thesubstrate and parallel to a source to drain direction to thesemiconductor islands 5237, the tunnel dielectric 5249, the floatinggates 5247 and the control gates 5239, as shown in FIG. 83B. The firstrail stacks 5271 are converted into islands during the etching of thesecond rail stacks 5235.

[0365] A second insulating layer 5257 is then deposited over the secondrail stacks 5235 and planarized by CMP to be level with the second railstacks, as shown in FIG. 83B. An interlayer insulating layer 5259 isthen deposited over the second insulating layer 5257 and the second railstacks 5235. If desired, a single insulating layer may be depositedabove and between the second rail stacks 5235 to form the secondinsulating layer 5257 and the interlayer insulating layer 5259. Thesingle layer is then planarized by CMP.

[0366] If desired, a plurality of additional device levels of the arraymay be monolithically formed above layer 5259 to form a threedimensional monolithic array having at least three device levels, asshown in FIG. 83A. Each device level is preferably separated by aninterlayer insulating layer.

[0367] In an alternative method of making the flash memory array, theword line in each device level may be formed below the bit lineconductors (i.e., bottom gate TFT EEPROMs rather than top gate TFTEEPROMs are formed). In the alternative method, the second rail stacks5235 comprising the gate lines 5239, the charge storage regions5251/5247/5249 and the semiconductor islands 5237 are formed first, asshown in FIG. 86E. Then, the first insulating layer 5245 is formed onthe semiconductor islands of the second rail stacks 5235. The firstinsulating layer 5245 may also be formed between the second rail stacksif desired. Alternatively, another insulating layer is formed betweenthe second rail stacks and planarized by CMP prior to the formation ofthe first insulating layer 5245.

[0368] Trenches are then formed in the first insulating layer 5245.Source and drain regions 5243 are formed in the semiconductor islands5237 by ion implanting (or diffusing) dopant ions through the trenches.The photoresist layer (not shown) used during the etching of thetrenches may be removed before or after the ion implantation. A secondconductive layer (such as a layer comprising polysilicon and silicidesublayers) is formed in the trenches and over the first insulatinglayer, as shown in FIG. 86F. The second conductive layer is thenplanarized by CMP to form the bit line conductors 5233 overlying thesemiconductor islands 5237. Alternatively, the source and drain regions5243 may be formed by outdiffusion from the bit line conductors 5233rather than by ion implantation.

[0369] Similar methods may be used to form the flash memory array havingTFT EEPROMs with an access transistor, as shown in FIG. 84 or having TFTEEPROMs with a drain offset region, as shown in FIG. 85. In thesemethods, the stack of layers which includes a tunnel dielectric layer5249 and a floating gate layer 5247 are deposited over the firstsemiconductor layer 5237, as shown in FIG. 86C. The stack of layers isthen patterned to form first rail stacks 5271 which includesemiconductor rails 5237 having a first width and charge storage regionrails 5247/5249 having a second width smaller than the first width, suchthat the first rail stacks have one aligned lateral edge and drainportions of the semiconductor rails 5237 are exposed.

[0370] Such a structure may be achieved by two different etchingmethods. The first etching method includes forming a first photoresistmask 5275 having a first width over the stack, as shown in FIG. 86G. Thefirst semiconductor layer 5237, the tunnel dielectric layer 5249 and thefloating gate layer 5247 are then etched using the first photoresistmask 5275, as shown in FIG. 86G. A second photoresist mask 5277, havinga second width smaller than the first width, is then formed over thefloating gate layer 5247. The tunnel dielectric layer 5249 and thefloating gate layer 5247 but not the first semiconductor layer 5237 arethen etched using the second photoresist mask as shown in FIG. 86H.

[0371] The second etching method includes forming a first photoresistmask 5279 having a first width over the stack and etching the tunneldielectric layer 5249 and the floating gate layer 5247 using the firstphotoresist mask 5279 to expose a portion of the first semiconductorlayer 5237, as shown in FIG. 86I. Then a second photoresist mask 5281,having a second width larger than the first width, is formed over thefloating gate layer 5247 and over an exposed portion of the firstsemiconductor layer 5237 (it is possible that there may be somemisalignment between layer 5281 and layers 5249/5249). The firstsemiconductor layer 5237 is then etched using the second photoresistmask 5281, as shown in FIG. 86J.

[0372] To form the TFT EEPROMs with an access transistor 5261 of FIG.84, a control gate dielectric layer 5251 is formed over the patternedfloating gates 5247 and over the exposed portions of the semiconductorrails 5237 of the first rail stacks 5271. The control gate dielectriclayer 5251 functions as a gate dielectric of the access transistor 5261over the exposed portions of the semiconductor rails 5237.

[0373] To form the TFT EEPROMs with a drain offset region 5267 of FIG.85, the control gate dielectric layer 5251 is patterned at the same timeas the floating gate layer 5247 and the tunnel dielectric layer 5249, toexpose the drain portion and part of the channel silicon of thesemiconductor rails 5237. A second insulating layer 5269 is then formedover the control gate dielectric 5251 and the exposed portion of thesemiconductor rails 5237, as well as between the semiconductor rails5237 to isolate the semiconductor rails from each other. Layer 5269 isrelatively thick, having a thickness that is the same as or greater thanthe thickness of the charge storage regions 5241. Layer 5269 is thenplanarized by CMP to expose the top portion of the charge storageregions. The word line 5239 is then formed over the second insulatinglayer 5269 to form the offset regions 5267.

[0374] The nonvolatile, multiprogrammable flash memory array of thepreferred embodiment provides many-times programmable cells in acrosspoint (i.e., rail stack) array. FN tunneling is used for programand erase. This allows many cells to be written in parallel and provideshigh density, low power file storage. In addition, the cell sizes perlayer compare very favorably with cell sizes of commercially availableflash memories.

[0375] V. CMOS Array for Logic and Memory Circuits

[0376] In the previous embodiments, arrays of NMOS or PMOS devices weredescribed. However, in another preferred embodiment of the presentinvention, an array of CMOS (complementary metal oxide semiconductor)transistors is provided. Preferably, adjacent NMOS and PMOS transistorshave a common gate. However, the adjacent NMOS and PMOS transistors mayhave separate gates if desired. The array of CMOS devices may comprisean array of vertical pillar CMOS devices, an array of self aligned CMOSTFTs or an array of rail stack TFTs, as described in any previousembodiment. The CMOS devices are preferably formed as a threedimensional monolithic array above the substrate. However, the CMOSdevices may also be formed in a two dimensional array in or above asemiconductor substrate, if desired.

[0377] The NMOS and PMOS transistors of the CMOS array may be formedadjacent to each other in the same device level in an alternatingfashion (i.e., as alternating NMOS and PMOS transistors). However, in apreferred embodiment of the present invention, the one charge carriertype transistors (i.e., NMOS or PMOS) are formed above the other chargecarrier type transistors (i.e., PMOS or NMOS) with a common gate line(also known as a word line in memory devices) between them. Thus, thearray preferably comprises a plurality of vertically stacked, commongate CMOS transistors.

[0378]FIG. 87 illustrates one device level of a vertically stacked,common gate CMOS array in a rail stack configuration according to apreferred embodiment of the present invention. It should be noted thatthe array may also be arranged in a self aligned TFT or pillarconfigurations described previously. The CMOS array in FIG. 87 issimilar to the array illustrated in FIG. 73, except that transistors ofdifferent charge carrier type are formed on either side of the gateline. In FIG. 87, the NMOS transistors are arranged below the PMOStransistors. However, it should be understood that the PMOS transistorsmay be arranged below the NMOS transistors if desired.

[0379] In FIG. 87, the array of CMOS devices 5300 is preferably formedover a planarized interlayer insulating layer 5301, such as a CMPplanarized silicon oxide layer. Layer 5301 is formed over a substrate(not shown) as in the previous embodiments. Each CMOS device is thus aCMOS TFT because it is formed over an insulating layer. However, theCMOS devices may be formed in a monocrystalline silicon substrate, ifdesired.

[0380] The array includes a plurality of gate lines (i.e., word lines)5303 (only one gate line is shown in the cross sectional view of FIG.87). Preferably the gate line comprises a first N+ polysilicon layer5305, a silicide layer 5307, such as a TiSi_(x) or WSi_(x) layer, overthe first polysilicon layer and a second P+ polysilicon layer 5309 abovethe silicide layer. The gate line 5303 acts as a gate electrode in eachTFT. Thus, no separate gate electrodes connected to the gate lines arerequired.

[0381] A first insulating layer 5311 is disposed adjacent to a firstside of the gate electrode 5303. This insulating layer 5311 may be aconventional gate dielectric. Preferably, the insulating layer 5311 is acharge storage layer (i.e., charge trapping media), such as an ONO stackor isolated nanocrystals, to form charge storage CMOS TFTS, such asEEPROM CMOS TFTs. If floating gate type EEPROM CMOS TFTs are desired,then a floating gate and a control gate dielectric may be added betweenthe insulating layer 5311 and the gate line 5303.

[0382] A p− type semiconductor layer 5313, such as a P− polysiliconlayer, is disposed on a side of the first insulating layer opposite tothe gate 5303. This layer contains the NMOS TFT bodies. N+ source anddrain regions 5315 are disposed in layer 5313. The portions of layer5313 between regions 5315 comprise NMOS TFT channel regions.

[0383] Preferably, the source and drain regions 5315 are formed byoutdiffusion of n− type dopants from the source and drain electrodes(i.e., bit lines) 5317. However, regions 5315 may be formed by any othermethod, such as by masking and ion implantation. The electrodes 5317contact the source and drain regions 5315 and are disposed on the bottomof the p− type semiconductor layer 5313 (i.e., on the side of layer 5313opposite to the first insulating layer 5311). Preferably, the electrodes5317 comprise N+ polysilicon rails which extend in a directionperpendicular to the gate line 5303. If desired, an optional metal ormetal silicide layer is formed in contact with electrodes 5317 toincrease their conductivity. However, the electrodes 5317 may comprisemetal or metal silicide instead of the heavily doped polysilicon, ifdesired. A planar insulating filler layer 5318, such as silicon oxide,is disposed between the source and drain electrodes 5317.

[0384] Thus, each NMOS TFT 5319 is located between adjacent source anddrain regions 5315 and comprises a portion of layers 5305, 5311, 5313and 5317, as illustrated in FIG. 87. The PMOS TFTS 5321 are locatedabove the NMOS TFTs 5319.

[0385] The PMOS TFTs 5321 include a second insulating layer 5323adjacent to a second side of the gate electrode 5303. In FIG. 87, layer5323 is located on the P+ polysilicon layer 5309 of the gate line 5303.The insulating layer 5323 may be a conventional gate dielectric.Preferably, the insulating layer 5323 is a charge storage layer (i.e.,charge trapping media), such as an ONO stack or isolated nanocrystals,to form charge storage CMOS TFTS, such as EEPROM CMOS TFTs. If floatinggate type EEPROM CMOS TFTs are desired, then a floating gate and acontrol gate dielectric may be added between the insulating layer 5323and the gate line 5303.

[0386] An n− type semiconductor layer 5325, such as an N− polysiliconlayer, is disposed above the second insulating layer 5323. Layer 5325 isdisposed on the opposite side of layer 5323 from the gate electrode5303. P+ source and drain regions 5327 are disposed in layer 5325, suchthat regions of layer 5325 between the source and drain regions 5327comprise channel regions of PMOS TFTs. Source and drain electrodes 5329are disposed over the N− polysilicon layer 5325 and in contact with thesource and drain regions 5329. Thus, the electrodes 5329 are disposed ontop side of the N-polysilicon layer 5325 opposite to the secondinsulating layer 5323. A planar insulating filler layer 5331, such assilicon oxide, is disposed between the source and drain electrodes 5329.If desired, an optional metal or metal silicide layer is formed incontact with electrodes 5329 to increase their conductivity.

[0387] Thus, each PMOS TFT 5321 is located between adjacent source anddrain regions 5327 and comprises a portion of layers 5309, 5323, 5325and 5329, as illustrated in FIG. 87. A TFT EEPROM CMOS device (5319 and5321) is formed at each intersection of the first and the thirdspaced-apart electrodes or conductors 5317, 5329 and the common gateline 5303. If desired, the CMOS structure may be inverted and the PMOSTFTs formed below NMOS TFTs. It should be noted that NMOS and PMOSelectrodes (i.e., bit lines) do not have to fall directly on top of eachother, although they preferably should have the same pitch. NMOS andPMOS transistors thus can have different channel lengths, but the pitch(and thus array size) will be limited by the longer of the two channellengths. In one preferred aspect, TFTs of one conductivity type (i.e.,NMOS or PMOS TFTs) contain a charge storage layer or region, while TFTsof the other conductivity type (i.e., PMOS or NMOS) do not have a chargestorage region or layer. Thus, the CMOS of this aspect comprises oneEEPROM TFT and one non-EEPROM TFT.

[0388] The TFT CMOS device array 5300 illustrated in FIG. 87 is highlyplanar and compact. The NMOS source and drain electrodes 5317 comprisepolysilicon rails which extend above the interlayer insulating layer5301 in a first plane parallel to the substrate surface. The p− typepolysilicon layer 5313 extends above the source and drain electrodes5317 in a second plane. The gate line 5303 extends above layers 5317,5313 and 5311 in a third plane. The n− type polysilicon layer 5325extends above the gate line 5303 in a fourth plane. The PMOS source anddrain electrodes 5329 comprise polysilicon rails which extend above then− type semiconductor layer 5325 in a fifth plane. Each of the fiveplanes does not intersect any of the other planes.

[0389] The TFT CMOS array 5300 is also self aligned. The gate electrode5303, the first insulating layer 5311, the p− type semiconductor layer5313, the second insulating layer 5323 and the n− type semiconductorlayer 5325 comprise a rail stack which is located in a plane parallel tothe substrate. The rail stack extends perpendicular to the source anddrain electrodes 5317, 5329. Thus, the gate electrode 5303, the firstinsulating layer 5311, the p− type semiconductor layer 5313, the secondinsulating layer 5323 and the n− type semiconductor layer 5325 are selfaligned in a plane perpendicular to the substrate and parallel to thesource to drain direction, as will be described in more detail below.

[0390] The TFT CMOS array 5300 is preferably arranged in a monolithicthree dimensional array comprising a plurality of device levelsvertically separated by one or more interlayer insulating layers. Eachdevice level the array contains TFT CMOS devices 5300, as in theprevious embodiments. A peripheral or driver circuit (not shown) isarranged in the substrate, preferably below the array and at least inpartial vertical alignment with the array, or alternatively, within orabove the array and at least in partial vertical alignment with thearray.

[0391] FIGS. 88A-D illustrate a method of making the rail stack TFT CMOSarray 5300 according to a preferred embodiment of the present invention.First, an N+ polysilicon layer is deposited and patterned to form thesource and drain electrodes or conductors 5317. An insulating layer5318, such as a silicon dioxide layer, is then deposited over andbetween the conductors 5317. Layer 5318 is then planarized by CMP toform a planarized block 5331, as shown in FIG. 88A. The top surfaces ofthe conductors 5317 are exposed in the top surface of the block.

[0392] A stack of layers is then deposited on the block 5332. Theselayers include the p− type polysilicon (or amorphous silicon) layer5313, the first insulating or local charge storage film 5311, the gatelayer 5303, the second insulating or charge storage film 5323 and the n−type polysilicon (or amorphous silicon) layer 5325. A photoresist mask(not shown) is then formed over this stack, and the stack of layers ispatterned to form a plurality of rail stacks 5333 (only one rail stack5333 is shown in FIG. 88B for clarity). The mask may be removed afterall the layers have been patterned. Since all of the layers in railstack 5333 are patterned during the same step, the layers in the railstack 5333 are self aligned in a plane perpendicular to the substrate(i.e., the sides of the rail stack 5333 are planar). The rail stacks5333 are disposed above the block 5332. The rail stacks extend in adifferent direction from the direction of the electrodes 5317.Preferably, the rail stack 5333 and the electrodes 5317 extend inperpendicular directions within the array, as shown in FIG. 88B.

[0393] An insulating layer 5331, such as a silicon oxide layer, is thendeposited over the rail stack 5333, such that it fills in the spaces5335 between the rail stacks 5333, as shown in FIG. 88C. Layer 5331 isthen planarized by CMP. A photoresist mask (not shown) is formed onlayer 5331, and parallel trenches 5339 are etched in layer 5331 usingthe mask. The trenches extend parallel to the electrodes 5317 andperpendicular to the rail stacks 5333, as shown in FIG. 88C.

[0394] If desired, optional sidewall spacers (not shown) are formed onthe sidewalls of the rail stack 5333 before the deposition of layer5331. Preferably, the spacers are made from an insulating material thatis different from the material of layer 5331. The spacers are preferablymade of silicon nitride. The spacers protect the sidewalls of the stack5333 during the etching of the trenches. The spacers keep the trenchetch from extending too far past the top of the gate lines in the areabetween gate lines, to protect against gate to source/drain shorts.

[0395] Using layer 5331 and/or the photoresist as a mask, p− type ions(i.e., boron or BF₂) are implanted into the exposed n− typesemiconductor layer 5325 through the trenches 5339. The ions form P+source and drain regions 5327 in layer 5325, as shown in FIG. 88D.

[0396] A p− type polysilicon layer is then deposited over layer 5331 andin the trenches 5339. The polysilicon layer is planarized by CMP oretched back to form a plurality of spaced apart P+ electrodes 5329embedded in the planarized insulating layer 5331. The electrodes 5329are located above the rail stacks 5333 and contact the P+ source anddrain regions 5327. Since the electrodes 5329 and source and drainregions 5327 are formed during the same lithography step, there is nomisalignment between the electrodes 5329 and source and drain regions5327. Alternatively, the source and drain regions 5327 may be formed byoutdiffusion from the electrodes 5329 rather than by ion implantationinto the trenches 5339.

[0397] The array is annealed to form N+ source and drain regions 5315 byoutdiffusion from N+ electrodes 5317 and to recrystallize the amorphousor polysilicon semiconductor layers 5313 and 5325. The outdiffusion andrecrystallization may be carried out during the same or differentannealing steps at any desired point in the fabrication process.

[0398] If desired, an interlayer insulating layer is formed over thearray shown in FIGS. 87 and 88D, and another device level containinganother array of TFT CMOS EEPROM devices 5300 is monolithically formedthereon. Routing metallization layers (preferably a metal layer otherthan aluminum) may be formed in the interlayer insulating layer.Additional interlayer insulating layers and device levels may be formedover the second level of the array if desired, to form at least threedevice layers. In another alternative aspect of this embodiment, asecond rail stack containing a gate line is formed directly on top ofthe PMOS electrodes 5329 without an intervening interlayer insulatinglayer. Thus, the PMOS electrodes 5329 would contain source and drainregions in two rail stacks. In other words, plural device levels may beformed without intervening interlayer insulating layers to form a threedimensional monolithic array. This arrangement offers more transistorswith fewer processing steps, but with less programming flexibility.

[0399] As shown in FIG. 89, the resulting TFT CMOS array is a matrix ofNMOS 5319 and PMOS 5321 devices with common gates 5303. The array shownin FIG. 89 is an unprogrammed or unconfigured array. The array can thenbe configured into logic elements or memory devices by rupturing thegate dielectric (i.e., the charge storage film or region) to form aconductive link which connects the gate lines (i.e., word line rows)5303 and source and drain electrodes 5317, 5329 (i.e., bit lines), or bystoring charge in the charge storage regions of either NMOS or PMOStransistors to raise their threshold voltages and keep them permanentlyoff. The array of TFT CMOS EEPROM devices 5300 may be used to formeither logic elements or a memory array. Furthermore, the samesemiconductor device in the unconfigured array may be used either as anantifuse or as an EPROM or an EEPROM.

[0400] According to a preferred embodiment of the present invention, acircuit comprising a plurality of charge storage devices and a pluralityof antifuse devices is provided. The circuit may comprise a fieldprogrammable gate array or a programmable logic device. Preferably, theplurality of charge storage devices and the plurality of antifusedevices comprise a same set of devices. This greatly simplifies thefabrication of the circuit. These devices function as charge storagedevices when a first programming voltage is applied to the devices toturn these devices off by increasing their threshold voltage. Thesedevices also function as antifuses when a second programming voltagehigher than a first voltage is applied to the devices. The secondvoltage may be any voltage which is sufficient to form a conductive linkthrough the charge storage region. For example, the first (i.e., chargestorage voltage) may be less than 5 volts, while the second voltagesufficient to form the conductive link may be 5-50 volts, depending onthe device characteristics. The voltages are provided to the devices bythe driver or peripheral circuit. However, if desired, charge storageand antifuse semiconductor devices having a different structure may beprovided.

[0401] It should be noted that any charge storage devices which functionas an antifuse when a conductive link has been formed through its chargestorage region are within the scope of the present invention. Thus, anydevice is within the scope of the present invention if the devicecontains a semiconductor active region, a charge storage region adjacentto the semiconductor active region, a first electrode and secondelectrodes, and where charge is stored in the charge storage region whena first programming voltage is applied between the first and the secondelectrodes, and a conductive link is formed through the charge storageregion to form a conductive path between the first and the secondelectrodes. Therefore, a charge storage device which is capable of beingused as an antifuse is not limited to rail stack TFT EEPROMs. Suchcharge storage devices may include the pillar or self aligned TFTEEPROMs and diodes with charge storage regions of the previousembodiments, as well as EPROMs and EEPROMs formed in a single crystalsemiconductor substrates.

[0402]FIG. 90 illustrates how a 4×4 cell array of the circuit of FIG. 89can be programmed into an inverter 5343. First, a high voltage isapplied between gate (i.e., word) line 5345 and bit lines 5347, whichwill be used to carry the output voltage, V_(out). This causesconductive antifuse links 5348 to form to electrically connect lines5345 and 5347. Then, the driver circuit provides a programming voltageto all other transistors 5350 to increase their threshold voltage toturn them off, except to NMOS transistors 5355 and PMOS transistors5357. The NMOS 5355 and PMOS 5357 transistors form the inverter. When ahigh voltage, V_(in), is provided into gate line 5349, then a lowvoltage, V_(out), is read out, and vice-versa. Voltages V_(SS) (i.e.,ground) and V_(DD) (i.e., power supply voltage) are provided into bitlines 5351 and 5353 which are connected to transistors 5355 and 5357.

[0403]FIG. 91 illustrates how a 4×4 cell array of the circuit of FIG. 89can be programmed into a two input NAND gate 5360. First, a high voltageis applied between gate (i.e., word) line 5345 and bit lines 5347, whichwill be used to carry the output voltage, V_(out). This causesconductive antifuse links 5348 to form to electrically connect lines5345 and 5347. Then, the driver circuit provides a programming voltageto all other transistors 5350 to increase their threshold voltage toturn them off, except for PMOS transistors 5361 and 5365 and NMOStransistors 5363 and 5365. The transistors 5361, 5363, 5365 and 5367form the NAND gate. Input voltages V_(in1) and V_(in2) are provided intogate lines 5369 and 5371. CMOS 5361/5363 is connected to gate line 5369,while transistors 5365 and 5367 are connected to gate line 5371.Voltages V_(SS) and V_(DD) are provided into bit lines 5373 and 5375.NMOS 5367 is connected to bit line 5375, while PMOS 5361 and 5365 areconnected to bit line 5373. Output voltages can be read out from lines5345 or 5347, which are connected by a blown antifuse 5348.

[0404]FIG. 92 illustrates how a 5×6 cell array of the circuit of FIG. 89can be programmed into a static random access memory (SRAM) 5380. First,a high voltage is applied between gate (i.e., word) lines 5381 and 5383and bit lines 5385, 5386, 5387 and 5388. This causes conductive antifuselinks 5348 to form to electrically connect lines 5381 with lines 5385and 5386, and to electrically connect lines 5383 with lines 5387 and5388. Then, the driver circuit provides a programming voltage to allother transistors 5350 to increase their threshold voltage to turn themoff, except for transistors 5389, 5390, 5391, 5392, 5393 and 5394. Thetransistors 5389 and 5390 are the SRAM access transistors, whiletransistors 5391, 5392, 5393 and 5394 are the cross coupled inverters.The cell is accessed by placing a positive voltage on the word line5395. Data is input onto and read out of BL and BL-bar, which areprovided into bit lines 5396 and 5397, respectively. Voltages V_(SS) andV_(DD) are provided into bit lines 5398 and 5399, respectively.

[0405]FIGS. 89-91 show various exemplary configurations that can beprogrammed. It should be noted that any other desired logic or memorydevice, such as a NOR gate, etc., may be programmed using the methodsdescribed above. Since all logic functions can be performed by basicelements, such as NAND gates, any logic circuit can be programmed intothis type of an array. Furthermore, logic and memory devices may beprogrammed into the same circuit if desired. For logic devices, ingeneral, the size of the logic block is (x+1)² times the cell area,where (x) is the number of inputs on the logic gate. Since the cell areahere can be as small as 4F², where F is the minimum feature size(half-pitch), then for F=0.25 microns, the minimum area per logic gateis 4(F(x+1))², or 2.25 microns squared for a 2-input NAND or NOR gate.Preferably, the area per logic gate is 4(F(x+1))² to 5(F(x+1))². Thissize includes an “isolation” row and column on each edge of the block,that is shared with the next block.

[0406] VI. Metal Induced Crystallization

[0407] A preferred embodiment of the present invention is directed to anon-volatile thin film transistor (TFT) memory or logic deviceconstructed above a substrate and including a source, drain and channelregion made of deposited or grown amorphous silicon or polysilicon thathas been crystallized by means of a transition metal-induced lateralcrystallization (MILC) process. A two- or, more preferably, athree-dimensional many-times programmable (MTP) non-volatile memory orlogic is constructed of such thin film transistor memory devices.

[0408] In accordance with the first aspect of the present embodiment, itis desirable to improve the performance characteristics of TFT-basednon-volatile memory or logic cells having a channel formed in adeposited thin layer of silicon, such as amorphous silicon (a-Si) orpolysilicon. This can be accomplished if the grain size of the a-Si orpolysilicon can be increased to resemble monocrystalline silicon.

[0409] In the past, crystallization of a-Si has been accomplished in anumber of ways. In accordance with a first approach, a-Si may bepartially crystallized to form polycrystalline silicon with an annealstep taking tens of hours at about 600° C. This approach is notadvantageous because the devices formed in that material havelower-performance characteristics and they take a relatively long amountof time to fabricate. Thus, crystallization can be enhanced by the useof transition metal or germanium catalysts to induce lateralcrystallization at seeding sites.

[0410] Unfortunately, most transistor-based devices fabricated in thismanner suffer from relatively poor performance characteristics (relativeto monocrystalline silicon) and exhibit subthreshold slope values on theorder of 100's of mV/dec and an Idsat of 10's of μÅ/μm. Themetal-induced lateral crystallization (MILC) is carried out at atemperature of about 400° C. to about 700° C. to achieve lateralcrystallization growth rates of several or more μm/hr. To furtherenlarge the silicon crystal sites to hundreds of microns, a relativelyshort duration high temperature anneal step, e.g., 900° C. for 30minutes, is added to simultaneously crystallize multiple layers of a-Si(or another semiconductor material). Note that a crystallizationtemperature range of about 750° C. to about 975° C. will also providesatisfactory results if the time of the anneal is adjusted accordingly.This short duration high temperature anneal will not saturate thediffusion regions of the devices contemplated herein and can be appliedonce to a multi-level device, as can the low temperature anneal step.

[0411] An example of a process for recrystallizing a deposited a-Silayer in accordance with a specific embodiment of the present inventionis now described and illustrated in FIGS. 93-95. Those of ordinary skillin the art will now realize that many routine modifications to theprocess illustrated here are possible and do not affect the inventiveconcepts set forth herein.

[0412] Turning now to FIGS. 93-95, a process flow diagram of afabrication process for a crystallized deposited (or grown) a-Si layeris illustrated in FIG. 93. FIGS. 94A-94H illustrate vertical crosssections of a silicon wafer prepared in accordance with the process ofFIG. 93. FIG. 95 illustrates the effect of metal-induced lateralcrystallization (MILC) through seeding windows 5424 in a-Si depositedover buried oxide over a standard silicon wafer.

[0413] The first step 5406 of the process 5408 is to grow (or deposit) athick oxide layer 5410 (FIG. 94A) (e.g., 3000 Å) on a standard siliconwafer substrate 5412 to provide a buried oxide layer. The next step 5414is to deposit a thin amorphous silicon (a-Si) layer 5416 (e.g., 1000 Å)over buried oxide layer 5410. This can be accomplished, for example,with low pressure chemical vapor deposition (LPCVD) at 550° C. usingSiH₄ as the silicon source at a flow rate of 70 SCCM and a pressure of300 mtorr. Alternatively, layer 5416 may comprise a polysilicon layer.The next step 5418 is to deposit a sacrificial low temperature oxide(LTO) layer 5420 (e.g., 3000 Å) and then in step 5419 to pattern it withmask 5422 and etch to expose transition metal seeding widows 5424. Theseseeding windows can be slots approximately 2 μm in width as shown inFIG. 95. Mask 5422 can now be removed.

[0414] The next step 5426 is to deposit a transition metal layer 5428(e.g., 100 Å Ni (nickel)) over LTO layer 5420. Other transition metalsmay be used although Ni is presently preferred. Other transition metalswhich may also be used, but which are less desirable than Ni are: Fe(iron), Co (cobalt), Ru (ruthenium), Rh (rhodium), Pd (palladium), Os(osmium), Ir (iridium), Pt (platinum), Cu (copper) and Au (gold).Germanium may also be used if desired. The transition metal may also beintroduced into the seeding window by implantation and other mechanismswell known to those of ordinary skill in the art.

[0415] The next step 5430 is to anneal for initial lateralcrystallization. This step, illustrated in FIG. 94F, may be carried outin a range of temperature and times. For example, a 20 hour anneal at560° C. in N₂ ambient will work. Lower temperatures require longeranneal times, higher temperatures require shorter anneal times. Those ofordinary skill in the art will now recognize that this can be optimizedfor throughput considerations. This step performs a crystallizationwhich may be adequate for certain devices and provide silicon grainsizes of several to tens of μm. Other devices requiring even moreperformance and silicon grain sizes in the hundreds of μm may requirethe high temperature anneal step discussed below.

[0416] The next step 5432 is to strip the remaining transition metallayer 5428. This may be performed with H₂ SO₄:H₂O₂ (4:1) at 70° C. Thenstep 5434 is the LTO layer 5420 is stripped with HF.

[0417] Finally, a high temperature anneal step 5436 (e.g., 900° C., 30minutes, N₂ ambient) is conducted (if desired) to further crystallizethe partially crystallized a-Si to form even larger grain siliconcrystals, (>100 μm in size). This step gives the crystallized a-Si layer(i.e., a large grain polysilicon layer) performance characteristicssimilar to conventional SOI (silicon on insulator) CMOS technology. Notethat transition metal-crystallized semiconductor material as used hereinwill contain trace detectable amounts of the transition metal(s) usedfor facilitating the crystallization. In normal semiconductorprocessing, trace amounts of transition metals (typically Fe, Ni) willescape the structure of the semiconductor fabrication equipment (usuallycontaining stainless steel) and embed themselves into the semiconductorfilm where the TFT channel would be formed. Normally these transitionmetals are present at a level of less than about 10¹⁴ atoms/cc. Intransition metal crystallization, however additional trace amounts oftransition metals in excess of about 10¹⁴ atoms/cc and up to about 10¹⁸atoms/cc will remain in the crystallized semiconductor material afterprocessing. This is generally not a contamination problem, however,where it is desired to create a gradient of such contaminants, agettering material, e.g., P (phosphorous), may be placed in the sourceand/or drain regions of the TFT to reduce the concentration of suchcontaminants in the channel region by increasing the concentration ofsuch contaminants in the respective source and/or drain regions.Formation of devices in the region of the seeding windows 5424 should beavoided due to excessive transition metal contamination.

[0418] The above described metal induced crystallization method may beused to recrystallize the active semiconductor layer of any of the abovedescribed devices. Thus, pillar TFTs, self-aligned TFTs, rail stack TFTsand diodes (i.e., an active semiconductor layer which contains one ormore p-n junctions) of various configurations may be formed in therecrystallized a-Si or polysilicon.

[0419] VII. Metallization

[0420] In the various embodiments described above, a metal silicidelayer was formed in contact with a silicon layer, such as a polysiliconword line or bit line. One preferred method of forming a titaniumsuicide layer in contact with a silicon layer is by using a silicon capand a TiN layer. The titanium silicide layer is formed on an undopedamorphous silicon cap layer. The cap layer is formed on a heavily dopedsilicon layer, such as a polysilicon or amorphous silicon layer doped toa concentration in excess of 10¹⁹ cm³, such as 10¹⁹ cm⁻³ to 10²¹ cm³.The cap layer is preferably deposited on P+ polysilicon or N+ amorphoussilicon layers. The N+ amorphous silicon may then be recrystallized intoN+ polysilicon during subsequent annealing steps.

[0421] A method of forming a titanium silicide (TiSi₂) layer comprisesthe following steps. A heavily doped polysilicon layer is deposited. Forexample, a P+ polysilicon layer is boron doped to a concentration of5×10²¹ cm³, and has a thickness of about 1400 Angstroms. A cap layer ofundoped amorphous silicon is deposited on the P+ polysilicon layer. Thecap may be 600 Angstroms thick, for example. A titanium layer isdeposited on the cap. The titanium layer may be 250 Angstroms thick, forexample. A titanium nitride layer is deposited on the titanium layer.The titanium nitride layer may be 100 Angstroms thick, for example.Other layer thicknesses may be used, as required.

[0422] The layers are annealed at a temperature below 650° C. for lessthan five minutes to react the titanium and the silicon in the cap toform a C49 phase TiSi₂ layer. The anneal may be carried out at 600° C.for 1 minute, for example. If desired, another P+ polysilicon layer isdeposited over the stack and the stack is etched into a thin “wire” or“rail”, such as a word line or bit line. The wire or rail may be 0.25 mmwide or less. The titanium silicide is then transformed from the C49 tothe C54 phase by a high temperature (i.e., above 650° C.) anneal. Theanneal can take place before or after the wires or rails are patterned,at 800° C. for one minute, for example. By annealing each Si/Ti/TiN filmstack below 650° C., dopant diffusion and thermal grooving of the TiSi₂is minimized. Multiple film stacks can be deposited and etchedsequentially.

[0423] The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedrawings and description were chosen in order to explain the principlesof the invention and its practical application. The drawings are notnecessarily to scale and illustrate the arrays in schematic blockformat. It is intended that the scope of the invention be defined by theclaims appended hereto, and their equivalents.

1-482. (Canceled).
 483. A monolithic three dimensional array comprising:a) a first device level of charge storage transistors, the first devicelevel comprising: i) a plurality of spaced apart first rails extendingin a first direction; and ii) a plurality of spaced apart second railsformed over the first rails extending in a second direction differentfrom the first direction, wherein the first rails comprise firstportions of the charge storage transistors and the second rails comprisesecond portions of the charge storage transistors; and b) a seconddevice level of charge storage transistors monolithically formed abovethe first device level.
 484. The array of claim 483, wherein the seconddevice level comprises: a plurality of spaced apart third railsextending in the first direction; and a plurality of spaced apart fourthrails formed over the third rails extending in the second direction;wherein the third rails comprise first portions of the charge storagetransistors of the second device level and the fourth rails comprisesecond portions of the charge storage transistors of the second devicelevel.
 485. The array of claim 483, wherein the first device level isdisposed above a monocrystalline semiconductor substrate.
 486. The arrayof claim 485, wherein driver circuits are disposed in themonocrystalline semiconductor substrate.
 487. The array of claim 483,wherein: the array comprises at least four device levels containing thecharge storage transistors; and adjacent device levels are separated bya respective interlevel insulating layer.
 488. The array of claim 483,further comprising: an interlevel insulating layer located between thefirst and second device levels; a via having a bottom end, the viaextending between the first and second device levels through theinterlevel insulating layer; and a contact which extends between thefirst and second device levels in the via to contact an active region inone of the first rails at the bottom end of the via.
 489. The array ofclaim 483, wherein: each first rail comprises a plurality of polysiliconactive regions and each of the plurality of active regions is in contactwith a charge storage region.
 490. The array of claim 489, wherein: thefirst direction is substantially perpendicular to the second direction;and each of the charge storage regions comprises an O—N—O dielectricstack.
 491. The array of claim 490, further comprising at least twoconductive layers which contact each first rail.
 492. The array of claim491, wherein: the charge storage transistors are formed at intersectionsof each first rail and each second rail; each transistor active regioncomprises source, channel and drain regions; each transistor comprises agate electrode disposed in one of the second rails.
 493. The array ofclaim 492, wherein the second rails comprise word lines.
 494. The arrayof claim 483, further comprising a third device level of charge storagetransistors formed between the first and the second device levels. 495.A monolithic, three dimensional array of charge storage devices,comprising a plurality of device levels monolithically formed above asubstrate, wherein: each of the plurality of device levels comprises aplurality of spaced apart semiconductor active regions and a pluralityof spaced apart rails; the plurality of active regions are disposed at afirst height; the plurality of rails are disposed at a second heightdifferent than the first height; and wherein the active regions comprisefirst portions of the charge storage devices and the rails comprisesecond portions of the charge storage devices.
 496. The array of claim495, wherein: each charge storage device comprises a first portionlocated in one active region and a second portion located in one portionof one rail; each rail comprises a word line; and a charge storage filmis located in contact with the word line and in contact with theplurality of the active regions.
 497. The array of claim 495, whereinthe substrate is a monocrystalline semiconductor substrate.
 498. Thearray of claim 495, wherein driver circuits are disposed in thesubstrate.
 499. The array of claim 496, wherein a space between saidspaced apart rails is filled with an insulating material.
 500. The arrayof claim 496, wherein: the active regions comprise polysilicon; and thecharge storage film comprises an O—N—O dielectric stack.
 501. The arrayof claim 496, wherein: the charge storage devices are located in atleast four device levels; and adjacent device levels are separated by arespective interlevel insulating layer.
 502. The array of claim 496,further comprising at least two conductive layers which contact eachactive region.
 503. The array of claim 502, wherein: the charge storagedevices comprise charge storage transistors; source, channel and drainregions of each transistor are located in each active region; and eachtransistor gate electrode comprises a portion of the word line whichintersects the transistor active region.
 504. The array of claim 503,wherein: the semiconductor active regions are disposed in elongatedsemiconductor strips having two vertical side surfaces and a horizontalsurface which contacts the charge storage film; and the rails compriseelongated strips having two vertical side surfaces and a horizontal wordline surface.
 505. The array of claim 496, further comprising: aninterlevel insulating layer located between first and second devicelevels of the plurality of device levels; a via having a bottom end, thevia extending between the first and second device levels through theinterlevel insulating layer; and a contact which extends between thefirst and second device levels in the via to contact an active region atthe bottom end of the via.
 506. The array of claim 504, wherein thecharge storage devices are located at intersections of the elongatedsemiconductor strips and the rails.
 507. A method of making an array ofcharge storage transistors disposed above a substrate, comprising: a)forming a first device level, the first device level comprising: i) aplurality of spaced apart first rails extending in a first direction;and ii) a plurality of spaced apart second rails formed over the firstrails extending in a second direction different from the firstdirection; wherein the first rails comprise first portions of the chargestorage transistors and the second rails comprise second portions of thecharge storage transistors; and b) monolithically forming a seconddevice level of charge storage transistors above the first device level.508. The method of claim 507, wherein the second device level comprises:a plurality of spaced apart third rails extending in the firstdirection; and a plurality of spaced apart fourth rails formed over thethird rails extending in the second direction; wherein the third railscomprise first portions of the charge storage transistors of the seconddevice level and the fourth rails comprise second portions of the chargestorage transistors of the second device level.
 509. The method of claim507, wherein: the step of forming the first rails comprises: forming asemiconductor layer above a substrate; and patterning the semiconductorlayer into a plurality of elongated strips having side walls and anupper surface; and the step of forming the second rails comprises:forming a word line film on a charge storage film; and patterning theword line film to form the second rails.
 510. The method of claim 507,wherein: the step of forming the second rails comprises: forming a wordline film over a substrate; and patterning the word line film to formthe second rails; and the step of forming the first rails comprises:forming a semiconductor layer on a charge storage film above the secondrails; and patterning the semiconductor layer into a plurality ofelongated strips having side walls and an upper surface.
 511. The methodof claim 507, further comprising: monolithically forming at least fourdevice levels over each other; and forming interlevel insulating layersbetween adjacent device levels.
 512. The method of claim 507, furthercomprising forming an insulating material between adjacent second rails.513. The method of claim 507, wherein: each first rail comprises aplurality of polysilicon charge storage transistor active regions; eachsecond rail comprises a word line; and the first direction issubstantially perpendicular to the second direction.
 514. The method ofclaim 513, further comprising: forming at least two conductive layerswhich contact each first rail; and forming a charge storage film whichcontacts the active regions and the word line.
 515. The method of claim514, wherein: the charge storage film comprises an O—N—O dielectricstack; each transistor active region comprises source, channel and drainregions; and each transistor gate electrode comprises a portion of theword line which overlies the transistor active region.
 516. The methodof claim 515, further comprising: forming an interlevel insulating layerbetween the first and second device levels; forming a via through theinterlevel insulating layer which extends between the first and seconddevice levels to an active region in a first rail; and forming a contactin the via, such that the contact extends between the first and seconddevice levels to contact the active region at a bottom of the via. 517.The method of claim 515, wherein: the step of forming the first railscomprises forming a first photoresist mask over a semiconductor layerand patterning the semiconductor layer using the first photoresist mask;and the step of forming the second rails comprises forming a secondphotoresist mask over a word line layer and patterning the word linelayer using the second photoresist mask.
 518. The method of claim 507,wherein the charge storage transistors are located at intersections ofeach first rail and each second rail.
 519. The method of claim 507,further comprising forming a third device level of charge storagetransistors between the first and the second device levels.
 520. Anintegrated circuit, comprising: a three-dimensional memory array havingmore than one memory level of memory cells, each memory level of saidintegrated circuit comprising: a plurality of rails on an insulatinglayer above the substrate, said rails running in a first direction, saidrails comprising a lightly-doped semiconductor region disposed betweenheavily-doped semiconductor regions; a charge storage film on the rails;a plurality of word lines on the charge storage film, said word linesrunning in a second direction different than the first direction; and aninterlevel insulating layer above the word lines.
 521. The integratedcircuit of claim 520, wherein the integrated circuit further comprisesvias formed through at least one interlevel insulating layer makingcontact to portions of the rails.
 522. The integrated circuit of claim520, wherein the charge storage film comprises a charge storagedielectric film.
 523. The integrated circuit of claim 522, wherein thecharge storage dielectric film comprises silicon, oxygen and nitrogen.524. The integrated circuit of claim 523, wherein the charge storagedielectric film comprises a silicon oxide/silicon nitride/silicon oxide(ONO) stack.
 525. The integrated circuit of claim 520, furthercomprising a silicide layer formed on or within the word lines.
 526. Theintegrated circuit of claim 520, wherein the memory levels aremonolithically formed over the substrate.
 527. The integrated circuit ofclaim 520, wherein the lightly-doped semiconductor region comprises atransistor channel region and the heavily-doped semiconductor regionscomprise transistor source and drain regions.
 528. The integratedcircuit of claim 527, wherein the rails comprise polysilicon rails. 529.A method for manufacturing an integrated circuit memory array,comprising: forming a plurality of rails on an insulating layer above asubstrate, said rails running in a first direction; formingheavily-doped semiconductor regions in the rails; forming a chargestorage film on the rails; forming a plurality of word lines on thecharge storage film, said word lines running in a second directiondifferent than the first direction; forming an interlevel insulatinglayer above the word lines; and monolithically forming a second devicelevel over the interlevel insulating layer.
 530. The method of claim529, further comprising forming vias through the interlevel insulatinglayer.
 531. The method of claim 530, wherein the step of forming thevias comprises: forming openings in the interlevel insulating layer toexpose a portion of at least one rail or word line therebelow; andfilling the openings with a conductive material.
 532. The method ofclaim 529, wherein the charge storage film comprises a charge storagedielectric film.
 533. The method of claim 532, wherein the substrate ismonocrystalline semiconductor.
 534. The method of claim 533, wherein thecharge storage dielectric film comprises silicon, oxygen and nitrogen.535. The method of claim 534, wherein the charge storage dielectric filmcomprises a silicon oxide/silicon nitride/silicon oxide (ONO) stack.536. The method of claim 529, wherein the step of forming the pluralityof rails comprises: depositing a layer of silicon on the underlyinginsulating layer; and masking and etching the silicon layer to form theplurality of rails.
 537. The method of claim 536, wherein the depositedlayer of silicon comprises a polycrystalline silicon layer.
 538. Themethod of claim 529, further comprising the step of forming a silicidelayer on or within the word lines.
 539. The method of claim 538, whereinthe silicide layer is formed on a deposited polysilicon layer beforeetching the polysilicon layer to form the word lines.
 540. The method ofclaim 529, wherein: the heavily-doped semiconductor regions comprisetransistor source and drain regions; and lightly-doped semiconductorchannel regions are disposed between the source and drain regions. 541.The method of claim 529, wherein the second device level comprises aplurality of charge storage transistors.